The Geological Interpretation of a Gravity Survey of the Bristol Channel

The geological interpretation of a gravity survey of the Bristol Channel

MICHAEL BROOKS & MARTIN S. THOMPSON

CONTENTS x Summary of the known geology . 246 2 Bouguer anomalies in the Bristol Channel area 250 3 Geological interpretation . . 251 (A) Rock densities used in the interpretation 25I (B) Separation of the main anomalies 252 (c) The Lundy high 253 (D) The Bristol Channel low and the Exmoor gradient 257 (~.) The Bouguer anomaly field south and west of Gower 266 (v) The Bridgwater Bay high 266 (o) The regional field 268 4 Discussion 268 5 References 27I

SUM MARY A Bouguer anomaly map of the Channel and underlying, low density, Upper Palaeozoic surrounding land areas is presented and rocks which are probably preserved in a interpreted. The main features of the map are: structural basin similar to the South Wales (x) a negative anomaly approaching --2o m- coal basin but overthrust by the Devonian gal trending WNW across the southern half of succession of Exmoor. The parallel belt of the Channel and adjacent areas of west positive anomaly in the northern half of the Somerset and north Devon; (2) a broad Channel is interpreted as overlying an anti- flanking area of weak positive anomaly in the clinorial zone, characterized by Old Red Sand- northern part of the Channel which may stone at shallow depth, separating the South extend southeastwards across the Cothel- Wales coal basin to the north from the pos- stone fault into Bridgwater Bay; and (3) a tulated basin to the south. The positive local positive anomaly of over +2omgal anomaly around Lundy Island is attributed around Lundy Island. These anomalies are all to the effect of a large basic pluton of Tertiary superimposed on a strong regional gradient of age occurring at shallow depth, and the gravity +0.38 mgal km -x to the southwest. field of Lundy is compared with that of other The negative anomaly in the southern part Tertiary igneous centres. A sketch structural of the Channel is due partly to Mesozoic strata section across the Bristol Channel is presented. in the Bristol Channel syncline but partly to

RECENTLY the University College of Swansea has been engaged in geological and geophysical investigations into the geology and structure of the Bristol Channel. This work has involved extensive bottom sampling, continuous seismic profiling, seismic refraction, gravity and some magnetic surveying. The gravity survey was carried out with the Institute of Geological Sciences on a co-operative basis. The survey comprises about 125 ° km of traverse line using a LaCoste and Rom- berg stabilized platform shipboard meter (LaCoste et al. 1967) belonging to the

Jl geol. Soc. Lond. vol. x29, I973, pp. 245-274, x2 figs. Printed in Northern Ireland.

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Natural Environment Research Council (N.E.R.C.). The area east of 4°I5'w (Morte Point, Devon to western Gower, Glamorgan) was surveyed during April i97i by the authors using R.R.S. John Murray and the University College of Swansea research vessel Ocean Crest. The area to the west was surveyed from M.V. Researcher in May and June 197 I, during an Institute of Geological Sciences cruise. Data from the western and eastern areas, were shared by the Institute and the authors and the latter undertook the geological interpretation of data. The survey of the western area involved 75 ° km of traversing along twelve east-west lines at a spacing of 5 km and four north-south lines at a spacing of x2 kin; the eastern area, with 5oo km of traverse line, was covered by fourteen north-south lines and eight east-west or oblique lines (Fig. I2). Position fixing was by means of Decca Navigator, fixes being taken at intervals of 4 or 5 rain in the eastern area and I o min in the western area. Gravity measurements were tied to a value of 9812o2.3 mgal at a berth on the south side of Dock No. I, Barry, Glamorgan. Data were reduced by computer, corrections being applied for water depth, tidal height, Eotvos and latitude effects. The applied Bouguer correction effectively replaced the water layer (almost everywhere less than 5 ° m deep) by rock of density 2.67 g cm -3. The Eotvos correction was based simply on the separation and azimuthal relationship of the two Decca fix positions adjacent to the calculation point, and no smoothing process was applied. The latitude correction was determined using the International Gravity Formula of x93o. Bouguer anomaly values in Fig. x2 are given relative to a value of 981265.oo mgal at Pendulum House, Cambridge, in the normal British convention. A high accuracy was maintained throughout the survey. Weather conditions were uniformly good and the cross-coupling error, as indicated by the chart recorder of the gravity meter (LaCoste et al. x967) was always small. The main source of error was uncertainty in the Eotvos correction due to the erratic effects of strong tidal currents that characterize the Channel, especially the inner parts. The overall accuracy of the survey is indicated by a comparison of gravity values at 46 traverse intersection points: the average intersection error was I. I mgal (hence, ±0.6 mgal) and the maximum error was 3.2 mgal (:k:I-6 mgal). Gravity readings were integrated over a 3"5 min interval, equivalent to a 0.6 km section of ground. Consequently, anomalies involving significant changes of horizontal gravity gradient over shorter distances than this are attenuated, but in the present account only large-scale anomalies are considered, for which the effect of the integration produces a negligible distortion of shape.

I. Summary of the known geology The Bristol Channel separates two areas of markedly different geology (Fig. I). North of the Channel is an area of folded Upper Old Red Sandstone and Carboniferous rocks exhibiting severe faulting of Hercynian and later age. In the Vale of Glamorgan, the Upper Palaeozoic rocks are partially concealed under a veneer of flat-lying or gently folded Keuper, Rhaetic and Lower Liassic strata. South of the Channel, in north Devon, severely tectonized Devonian strata occupy the northern limb of the Cornubian synclinorium. Further south, in

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central Devon, Culm measures of Carboniferous age succeed the Devonian sequence• The structure of the Culm has not been fully worked out, but tectonic repetition by thrusting is believed to result in the juxtaposing of contemporaneous rocks of markedly different facies (Reading i965). In all probability, important thrusts also characterize the belt of Devonian strata. A belt of folded Carboniferous Limestone extends from the Vale of Glamorgan across the mouth of the Severn (giving rise to the islands of Flat Holm and Steep Holme) and continues eastwards into the Mendip Hills. Thus the Bristol Channel south of the Vale of Glamorgan occupies a structural position analogous to that of the Glastonbury syncline between the Mendip axis and the Q.uantock Hills (Fig. i). The first geological survey in the Bristol Channel was carried out by workers from the National Institute of Oceanography, University College London and Bristol University in the early i96o's using bottom sampling and geophysical profiling techniques (Donovan et aL I96z ; Donovan i963; Lloyd I963). These workers produced an unpublished geological map which showed the southern half of the Bristol Channel to be largely floored by a sequence of Jurassic strata estimated to be z6oo m thick and folded into a synclinal structure which they called the Bristol Channel syncline. The youngest strata recovered from the syncline are of Kimmeridgian age (Lloyd i963; D. J. Evans, pers. comm.). The syncline trends in an ~.s~. direction from the area northeast of Lundy Island and

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SOUTH WALES COALFIELD

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Horse s h i3~:~--~: Rocks _" .'." ~;--~.. ~ \ • c .X'F-_ ¢.. , • ° , • o • • • "" "Ex'mo~r.... Lu'~ BY ~'~\%., • • • • • z ~\~ • . '-.. ~ Cannington ".''•'~•2" :'.''''." E~rehdc~n " ~'~-:: Park \ • ".'. FQuantock \ ~e o • • • • • •

Fro. z. Geology of the Bristol Channel area.

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dies out in the vicinity of Minehead; according to Donovan et al. (I97I), it is not straight-forwardly continuous with the Glastonbury syncline, the latter lying en echelon to the north (Fig. I). The thickness of the Jurassic in the Bristol Channel syncline is comparable with the thickest Jurassic successions known from the land area of Britain, found in the Weald and south Dorset. Upper Cretaceous strata directly overlie New Red Sandstone and older rocks to the south and west of Cornwall (Hamilton & Blundell I97I), giving evidence of important tectonic events in the Bristol Channel area in pre-Upper Cretaceous times. Boomer and sparker surveys by the authors have shown that the Bristol Channel syncline is a strongly asymmetrical structure: dips in the northern limb are less than io ° and usually less than 5 ° whereas in the southern limb dips exceed 2o ° in many places (Brooks, in discussion of Donovan et al. i97 i). The northern limb of the main syncline is now known from profiling records to be interrupted by a series of subsidiary folds and minor flexures. An extensive programme of core, grab and dredge sampling carried out by Mr D. J. Evans of Swansea has confirmed and extended the earlier work regarding the disposition of the Jurassic strata on the sea bed. The authors estimate a Mesozoic thickness of I6oo m based on the width of outcrop and average dip of the sequence in the southern limb of the Bristol Channel syncline. However, an unresolved problem is the importance of strike faults which are evident on the seismic profiling records but normally involve an indeterminate throw. Moreover, the profiling results give no information on the underlying Triassic sequence, so that the maximum thickness of the Mesozoic sequence in the syncline is uncertain. Independent estimates of Mesozoic thickness have been made at various points in the Ghannel by means of sonobuoy seismic refraction surveys carried out by Mr D. G. James. The calculated depths to Palaeozoic are consistent with a maximum Mesozoic thickness of about I6OO m (Mr James, pers. comm.), indicating that there is no great thickness of Triassic rocks under the Bristol Channel syncline. To the south of the Mesozoic outcrops, a belt of Devonian strata continuous with that of north Devon crops out off the coast of north and northwest Devon and extends westward towards Lundy Island (Fig. x). The southeast corner of Lundy Island contains metamorphosed sediments correlated by Dollar (I94I) with the Morte Slates (Table i), but the rest of the island is composed of a composite granite intrusion (Dollar i94i ) which has yielded a Lower Tertiary age of 52-+-2 my (Miller & Fitch i962), together with numerous trachytic and doleritic dykes. Owen (I97i) suggested that Lundy Island may-occupy a horst structure with recent emergence resulting from movements along members of the group of northwesterly trending faults which characterize Devon and Cornwall (Dearman x963; Shearman x967). Shearman (I967) pointed out that Tertiary dykes on Lundy are affected by northwesterly trending faults and the latter must therefore be of post Lower Tertiary age. The Horseshoe Rocks, 5 km off Bull point, are a submarine outcrop of dolerite. The rocks exhibit a negative magnetic anomaly having a central trough with an amplitude of --5 ° gammas and symmetrical flanking peaks of x5 gammas,

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indicative of a reversely magnetized body. The overall width of the anomaly is about 0- 5 km. From a rock sample recovered by Mr D. J. Evans and the reversed magnetisation, the Horseshoe Rocks are interpreted as a sheared Tertiary dolerite dyke associated with the Lundy centre. Earlier references to the spilitic nature of the rocks (Donovan et al. i96i , Owen i97i ) appear to be unjustified. A gravity survey of southwest England by Bott et al. (I958) revealed a strong gravity gradient across Exmoor (Fig. I2). Following a suggestion by Falcon (in discussion of Cook & Thirlaway i952 ) that the northward fall of gravity across the nearby Q uantock Hills of Somerset was due to an underlying major thrust, the Exmoor gradient was interpreted as being caused by concealed Upper Palaeozoic rocks of low density extending under the Channel and partially overthrust by the Devonian sequence of north Devon. Without gravity data in the Channel, it was impossible to outline fully the geometry of the concealed Upper Palaeozoic rocks. The postulated thrust was interpreted as involving at least 14 km of n0rthe_._..rly movement. The main evidence for a major thrust is to be found in the Cannington inlier (Fig. I) described by Wallis (I924). Wallis postulated a major pre-Triassic fault to separate the Devonian and Carboniferous Limestone outcrops of the inlier. Bott et al. (~958) accepted an earlier suggestion of Ussher (I89x) that the fault was a thrust, and linked it with the postulated thrust required to explain the gravity gradient. The subcrop of this thrust was suggested to course westwards from the Cannington inlier beneath the Trias and the Bristol Channel almost to Porlock, from where it was presumed to diverge from the coastline. Webby (1966) subsequently argued that the stratigraphic gap between the Devonian and Car- boniferous Limestone sequences in the Cannington inlier could be explained by a fault with a throw of only a 'few thousand feet' and that if a major thrust were required by the geophysical evidence it probably lay to the north of the Carbon- iferous Limestone outcrop. A wide belt of Hercynian thrusts with northerly movements occupies southwest England. It extends from north of the Mendips, where klippen of inverted Car- boniferous Limestone rest on Coal Measures (Welch i933), through the Culm belt of central Devon (Reading I965) to the Lizard and Start boundary thrusts. Ramsbottom (i97o) suggested that the Cannington thrust may be the local representative of a continuous structure extending from southern Ireland, where movement is estimated to be about 25 km, through southern Britain to Belgium, where movement of at least 3 ° km has taken place (Fourmarier I933). Recently, Owen (I97i) suggested that the structure and physiography of the Bristol Channel are controlled by major Tertiary fault movements on wNw and northwesterly trending lines (op cit Fig. I). The northwesterly faults represent extensions of the major dextral wrench faults of southwest England (Dearman I963; Shearman I967) , including the Cothelstone, Timberscombe, Combe Martin and Sticklepath-Lustleigh faults. The main geological successions in the Bristol Channel area are shown in Table I. The thickness of the severely tectonized Devonian and Carboniferous successions of north Devon is difficult to estimate but it may approach I O ooo m

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TAB L~ I " Geological successions in the Bristol Channel Area North Devon Bristol Channel South Wales Jurassic: Kimmeridge Clay to Lower Lias Lower Lias Rhaetic Beds Rhaetic Beds Permo-Triassic Keuper Beds Keuper Beds Upper Culm tUpper Lower Culm JCarboniferous Coal Measures /Lower Millstone Grit Pilton Beds '~/Carboniferous Carboniferous Lst Baggy and ! Marwood Beds ~ Upper Grey Grits /,, Pickwell Down Plateau Beds ]upper O.R.S. Sandstone t Devonian Morte Slates Brownstones~ Ilfracombe Beds Senni Beds ~ Lower Hangman Grits ], Red Marls [ O.R.S. (-- Foreland Grits)~ °wer. Tilestones } Lynton Beds -~x)evoman

(Anderson & Owen 1968). The very different lithological succession north of the Channel undergoes marked regional variations of thickness, but the order of magnitude is indicated by the Carboniferous succession of West Glamorgan which totals over 4ooo m, and the Old Red Sandstone succession of Carmarthen- shire and Breconshire which is over 15oo m thick. From the geology of surrounding land areas and from geological work already carried out in the Bristol Channel, the main geological problems of the Channel are (i) the structural disposition of the Mesozoic succession; (2) the lithology and stratigraphic range of the underlying Upper Palaeozoic rocks (involving major differences of facies throughout the Devonian and Lower Carboniferous); (3) the nature and scale of Hercynian folding; (4) the relative importance of folding and faulting in the structural and physiographic evolution of the area; and (5) the regional significance of the Lundy complex. These problems are discussed below in the context of the Bouguer anomaly map.

2. Bouguer anomalies in the Bristol Channel area The only previously published gravity data from the Bristol Channel are by Bott & Scott (x964) and Davey (i97o). These data have not been incorporated into the present surveys. The Bouguer anomaly map (Fig. I2) includes information over Glamorgan and Monmouthshire taken from an unpublished Ph.D. thesis by M. D. Thomas, who worked at the University College of Swansea. The anomaly field in north Devon, west Somerset and on Lundy Island was taken from Bott et al. (I958). Data for the part of Somerset adjacent to the mouth of the Severn were kindly supplied by the Institute of Geological Sciences. The matching of anomaly

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contours across the coastline of the Channel is uniformly good and there is no indication of incompatability between the various surveys used in the compilation. In this regard, the only survey involving high ground near to the coastline is that of Bott et al. (1958) and their use of local densities in the Bouguer reduction has apparently succeeded in removing the gravity effect of rocks above sea level datum. A striking feature is the anomaly rise from northeast to southwest, from about --5 mgal in the mouth of the Severn to about +55 mgal immediately west of Lundy Island. Superimposed on this strong regional gradient, five main areas of distinctive anomaly pattern can be recognized: I. The northern half of the Channel, Gower and the Vale of Glamorgan with weak positive deflections of the anomaly field exhibiting a Hercynian trend. 2. The southern half of the Channel, marked by an elongate negative anomaly named the Bristol Channel low. 3. Exmoor with a strong gravity gradient previously noted by Falcon (op cit) and by Bott et al. (i958). 4- The Lundy area with a roughly circular gravity culmination with peak values exceeding +5 ° mgal (the Lundy high) and extending eastwards, in an attenuated form, into Barnstaple Bay. 5- Bridgwater Bay, which is occupied by a roughly circular anomaly with an amplitude of about i o regal (the Bridgwater Bay high). Northwesterly extensions of two major faults in southwest England, the Stickle- path-Lustleigh and Cothelstone faults, delineate some of the above gravity features. Thus the Sticklepath-Lustleigh line defines the eastern edge of the main Lundy high, and the Cothelstone line separates the Bristol Channel low from the Bridgwater Bay high. These anomalies are discussed in more detail in the following section.

3. Geological interpretation

(A) ROCK DENSITIES USED IN THE INTERPRETATION No densities have been measured by the authors, but many measurements on rocks from the Bristol Channel area have been made by other workers (Table 2). All the area densities in Table 2 fall in the surprisingly small range ofo. 12 g cm-*. However, certain rocks known to exist in the Bristol Channel area are not represented in the table but must considerably extend the above range. Upper Liassic and higher Jurassic sequences preserved in the Bristol Channel syncline comprise clays with thin sandstones (D. J. Evans, pers. comm.) and undoubtedly have a density significantly lower than the limestone-shale sequence of the Lower Lias of the Vale of Glamorgan. A thin Keuper sequence, dominantly composed of marls, crops out in the Vale of Glamorgan (c. I5o m; Pringle 1948 ) and off the north Devon coast (c. 250 m, estimated from seismic profiling records). The Keuper Marl is known to have a uniform density throughout much of Britain and the local sequence is likely to have a density close to that of the English

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TABLE 2" Summary of rock densities in the Bristol Channel Area

Density Rock Unit Locality (gcm -3) Reference

SOUTH WALES Lower Lias Vale of G|amorgan 2.60 Thomas (x 968) Pennant grit South Wales Coalfield 2"69 Cook & Thirlaway (x952) Pennant Measures South Wales Coalfield 2.62 Thomas & Brooks (t973) L. & M. Coal Measures South Wales Coalfield 2-66 Thomas & Brooks (,973) Coal Measures unspecified South Wales Coalfield 2"65 Whetton et al. (x 955) Lower Coal Series South Wales Coalfield 2.6I Cook & Thirlaway Millstone Grit South Wales Coalfield 2.66 Thomas & Brooks (z973) Carboniferous Limestone South Wales 2"7o Thomas & Brooks 0973) U. Old Red Sandstone South Wales 2.62 Thomas & Brooks (t973)

S OUTHVV'EST ENGLAND Culm Central Devon 2.61 Bott et al. (t958) Devonian North Devon 2.62 Bott et al. (t 958) (sandstones: 2"55 g cm-a; shales: 2"72 g cm-3). Granite Lundy Island 2-58 Bott et al. (t958)

Midlands sequence, namely 2.42 gem -s according to Parasnis (I952) and 2"45 gcm -3 according to Brooks (I966). The Tertiary igneous complex of Lundy Island (Dollar i94 I) contains basic dykes with probable densities in the range from 2-9 ° to 3.oo gcm -3. Attention is drawn to the high values of Coal Measure densities in the South Wales coalfield, from which no pronounced negative anomaly would be expected over the coalfield. Thomas and Brooks (i973) have established gravity traverses across the coalfield and concluded that the only significant negative anomaly, with a maximum amplitude of --5 mgal, is restricted to the southern part of the coalfield; they attributed this anomaly to a concealed body of anomalously low density (probably a highly porous sandstone unit) within the Car- boniferous sequence. The difference of density between arenaceous and argillaceous units of the Devonian succession of north Devon appears to cause local anomalies coincident with rock outcrops. Thus A1-Sadi (I967) was able to explain a residual negative anomaly 1 over the outcrop of the Pickwell Down Sandstone in terms of the density contrast between the formation and its envelope, on the assumption that the formation reduces its dip at depth or is truncated by a thrust plane dipping south at 7 ° under the area.

(B) SEPARATION OF THE MAIN ANOMALIES Prior to the geological interpretation it was necessary to separate the anomalies of interest from the strong regional background field and this was achieved with the

1 After a uniform-density Bouguer reduction to sea level and the removal of a regional gradient.

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aid of trend surface analysis. Trend surfaces up to the fifth-degree were produced by computer using a programme by O'Leary el al. (I 966). Trend surface analysis assumes that any observed distribution of data values comprises a regular trend and a superimposed random noise which shows up as residual departures from the trend. In geophysics it is often the residual departures rather than the main trends that provide the geological interest and usually they cannot be considered to be random. Gravity anomalies of geological interest normally show systematic departures of constant sign from the regional trend. In general, these departures influence the level of any calculated trend surface and thus their amplitude tends to be underestimated. Where the anomalies are sufficiently isolated and distinctive to be identified and delineated, a convenient method for the determination of the trend surface is to suppress the observed data from the area of the local anomaly and to compute the trend surface across the area of interest using data only from surrounding areas. The resulting trend surface is then not materially influenced by the local anomaly. This method has been adopted. The polynomial giving the best fit to the data is not necessarily the appropriate surface to use as a datum for local anomalies. Thus where local anomalies such as the Lundy high, the Exmoor gradient and the Bristol Channel low themselves cover wide areas, third and higher degree surfaces contain significant variations across the areas of interest with wave lengths less than the widths of the local anomalies. Geologically it is clearly unsound to express a local anomaly of any wave length against a regional field containing components of shorter wave length. The aim of a regional correction is normally to remove only the gravity features of large scale and this is best done by low degree trend surfaces. Conse- quently second degree surfaces were used to define three dimensional local anomalies but with the two-dimensional interpretation of gravity profiles across the Bristol Channel, the first and second degree trend surfaces were rejected in favour of simple uniform regional gradients selected to agree closely with the Bouguer anomaly field beyond the limits of the Bristol Channel residual anomaly.

(C) THE LUNDY HIGH The Lundy high (Fig. 2) was determined by expressing the Bouguer anomaly field against a second degree trend surface, the latter being computed after omitting gravity data from the Lundy and Barnstaple Bay areas. In its main develop- ment west of Lundy, the high has an arcuate form with the island at its centre of curvature. A peak value of about 23 mgal is reached IO km WNW of Lundy, whilst the island itself is marked by relatively low anomaly values in the range from 8 to I I mgal. A subsidiary area of high anomaly, with peak values exceeding 16 mgal, occupies Barnstaple Bay. The overall area of the anomaly is approximately 15oo km ~. The Sticklepath-Lustleigh fault system is presumed to extend across the area of the Lundy high (Owen 1971 ) and its likely course, shown on Fig. 2, coincides with a marked easterly attenuation of anomaly values. Across this zone, horizontal gravity gradients locally exceed :3 mgal km -1. The actual course of the fault has been picked at four points from seismic profiling records which show a zone of good reflectors (presumably representing well bedded

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Mesozoic or Tertiary strata) abruptly terminated southwestwards against a zone with no internal reflectors (interpreted as Devonian or Culm at outcrop). The main Lundy high coincides with a circular positive magnetic anomaly on the aeromagnetic map of Great Britain, Sheet 2 (Geological Survey of Great Britain i965). From the northern end of this anomaly a linear negative anomaly extends towards the northwest for a distance of at least 3o km. This negative anomaly was attributed by Cornwell (I97I) to a Tertiary dyke or dyke swarm following a further extension of the Sticklepath-Lustleigh fault system. There is no broad magnetic anomaly straightforwardly associated with the subsidiary gravity high in Barnstaple Bay although localized anomalies of 5o-Ioo gammas have been discovered, but not systematically mapped, in various parts of the Bay. Davey (i97o) suggested that the high gravity values may be due to basic gneisses, but the disposition of the Lundy high in relation to the Tertiary granites and dykes of Lundy Island and its association with a large magnetic anomaly centred on Lundy strongly suggest the presence of abundant Tertiary basic igneous material at depth. In area and amplitude, the gravity anomaly is strikingly similar to that found in association with the Carlingford Tertiary igneous centre in Ireland (Cook & Thirlaway I952 ) where a ring ofgabbro and eucrite intrusions surrounds a granophyre body (Richey i932 ). Gravity measurements over other Tertiary centres were reviewed briefly by McQuillin & Tuson (I963). They gave the following maximum Bouguer anomalies in mgals associated with individual centres Rhum 76, Skye 73, Mull 72, Carlingford 60, Ardnamurchan 42 and Arran 4I. The peak of 55 mgal west of Lundy is in the middle of the range of anomalies over major Tertiary centres. However, these values are somewhat misleading since they take no account of the prevailing level of the regional field. Around

//~ ~J~._~\OL_ \, ,?km RESIDUAL ANOMALIES /D,'/'~..~~-~~"~ ~' L~'" LUNDY AREA \ Co°,our ,°,ervo, 2re,o, I i " ~ ~'2

\ \\m ', h // ko'% / / /L_

FIG. 2. Residual anomalies in the Lundy area. Anomaly values are expressed against a second degree trend surface.

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Rhum the regional field level is about Io mgal, leaving a local anomaly in excess of 65 mgal which McQuillin and Tuson explained by a source rock with a density of at least 3.o5 gcm -3 extending to a depth of 15 km. The amplitude of the local anomaly in the Lundy area is little more than a third of that over Rhum. Interpretation of the Lundy high was carried out by computer using a programme for gravity interpretation of three dimensional structures developed by one of the authors (M. B.) but closely similar to a programme described by Cordell & Henderson (i968). The anomalous mass was approximated by a cluster of right-rectangular prisms built above, below or symmetrically about a pre-selected reference depth. The observed anomaly was matched by an iterative process involving a series of automatic adjustments to the model shape. Inter- preted prism clusters were taken to represent plausible first approximations to the shape of a basic intrusion below the Lundy area. The subsidiary positive anomaly in Barnstaple Bay was incorporated into the Lundy high for the model calculations. This assumption does not materially affect the resultant models in the immediate vicinity of Lundy Island and in the area of the main gravity anomaly west of Lundy. A density contrast of o. 3 g cm -3 was used throughout, equivalent to a difference between a normal gabbro density of 2.9-3. % (say 2"95 g cm -3) and an average Upper Palaeozoic density of 2-65 g cm -3. If a substantial amount of ultrabasic material occurs at depth, the models presented will be overestimates of the size of the concealed intrusion. Attempts to fit the Lundy high were made by building prismatic models on a 14 × 8 grid above reference depths of 3, 4, 5 and Io km. It is appreciated that any concealed basic pluton is unlikely to have a flat base, but in the absence of independent evidence on the shape of the hypothetical intrusion the gravity interpretation is restricted to simple considerations of the likely depth to, and the thickness of, the causal body. Table 3 given below which summarises the main results. All models except that with a I o km base level produce agreement between observed and calculated anomalies within 5 mgal, which is regarded as a tolerable discrepancy. Figure 3 illustrates a part of the prismatic model derived by using a reference depth of 3 km. This shows the main accumulation of basic rocks to lie west of Lundy Island and indicates a deficiency of basic rocks immediately under the island.

TABLE 3: Details of three dimensional interpretation of the Lundy high

Reference level in km.

3 4 5 Io Max. thickness of body (km) 2-46 2"85 3"32 6-86 Min. depth to body (km) o'54 I'x5 1.68 3-x4 Depth under Lundy I. (kin) 2"75 3"9t 4"95 9"97 Max. thickness in Barnstaple Bay (kin) x'9o 2"27 2"73 5"84 Min. depth under Barnstaple Bay (kin) I.Io I'73 2"27 4"I6 Largest error (mgal) 3"5 4"2 4"6 6.o R.M.S. error (regal) o.6 o'9 I-2 2"5

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A pronounced feature of all the models is a sharp increase in the depth to their top surface across the extension of the Sticklepath-Lustleigh fault syste m (Fig. 3). If this effect represents vertical displacement of the basic pluton due to post intrusion faulting, the indicated displacement (in the instance illustrated, between one and two km) is only a rough guide to the actual fault movement since the models do not incorporate a change in the reference depth of the prisms across the fault. There is evidence of vertical movement along the Sticklepath-Lustleigh fault during Tertiary times: although Dearman (I 963) stated that in the Dartmoor area the fault is essentially a dextral shear fault which has the effect of displacing the northwestern boundaries of the Dartmoor Granite by about I-5 km, Fasham (1971) concluded from a gravity survey that the western margin of the nearby Bovey Tracey basin of Oligocene age is defined by a related fault with a north- easterly downthrow of 7oo m. An alternative view of the significance of the Sticklepath-Lustleigh line in the Lundy area is that emplacement of the Tertiary igneous complex is in some way related to movements along the fault system, as could be the case with dyke intrusion along the fault further to the northwest (Cornwell 197 I). From Table 3, a substantial amount of basic rock is also indicated to underline Barnstaple Bay. However this result should be viewed with caution, for the absence of a broad magnetic anomaly from the overall area of the Barnstaple Bay high may indicate that this gravity anomaly has an entirely different cause

STIOKLEPATH- LUSTLEIOH LINE Y

w~ //.."

11,"

- / / / /

F[ o. 3. Three dimensional prismatic model of basic intrusion capable of explaining the Lundy high.

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from the main Lundy high. In this connection it may be noted that Fig. 4 shows an area of high gravity extending from Barnstaple Bay eastwards across Exmoor. In summary, the Lundy high is indicative of an underlying, major basic intrusion at shallow depth with a maximum thickness of between 2.5 km and 4"0 km in the area west of Lundy Island. More detailed work, including magnetic and seismic refraction surveys, are required before the actual shape of the intrusion can be more fully outlined.

(D) THE BRISTOL CHANNEL LOVe AND THE EXMOOR GRADIENT An initialcorrection for the regional background across the above anomalies was applied by expressing the Bougucr anomaly field against a second degree trend surface computed by omitting data from the southern half of the Bristol Channel and Exmoor. The smoothed residual anomaly map (Fig. 4) shows an elongate negative anomaly centred off the coast of north Devon and west Somerset and, flanking it to the south, a ridge of weak positive values extending ESE from Barn- staple Bay across the area south of Exmoor. The Exmoor gradient is considerably reduced after correction for the steep regional gradient towards the northeast. It is clear from Fig. 4 that the remaining gradient over Exmoor and the Bristol Channel low are two components of a single, major negative anomaly of ESE trend covering a large area of the Bristol Channel, north Devon and west Somerset. This anomaly extends as a strong feature towards the area north of Lundy but in the east it dies out towards the projected extension of the Cothelstone fault which in Somerset forms the western boundary of the Quantock Hills (Webby 1966). The linear extent of the main Bristol Channel anomaly permits its interpretation in terms of two dimensional geological structure using gravity profiles. Two cross-channel profiles across the anomaly have been investigated, along longitude lines 3°3o'w and 4°oo'w, in order to study variations in the subsurface structure along the strike of this major gravity feature. The Bouguer anomaly profile (Fig. 5) along longitude line 3°3o'w (the Porlock profile) lies close to that chosen by Bott et al. (1958 , Fig. I I) to illustratethe gravity gradient across Exmoor. Along the Porlock profile, Bouguer anomaly values fall northwards across Exmoor from a maximum of about 25 regal to a minimum of --2 mgal in the Bristol Channel and rise again to 6 mgal near the Welsh coast. The attempt to define the regional background field by trend surfaces of low degree is not entirely successful with the Porlock profile. The first and second degree surfaces leave residual positive values at the southern end of the profile which, with the second degree surface, exceed 4 mgal in places. Although the positive residuals might indicate rocks of anomalously high density under Exmoor, the simplyifying assumption is made that the residual gravity due to the near surface structure is entirely negative. In order to leave an entirely negative residual anomaly the firstdegree surface (Fig. 5) was arbitrarilyraised by about 3 regal at the southern end of the profile, resulting in a regional gradient of o.53 regal km -x. This arbitrary adjustment of the regional field in no way alters the main significance of the geological interpretation presented, although it does 4

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O- <1[

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o 5-~E-JO " OUJ ~ z~

_.lZ ~

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Z (I) n- rn

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affect the amplitude and width of the residual anomaly and hence the shape of the resultant geological models. Correction of the Bouguer anomaly profile on the basis of the adjusted regional gradient leaves a negative residual anomaly with a width of over 3o km and an amplitude of --i o mgal. The gravity profile along longitude line 4°oo'w, (Fig. 6), is referred to as the Combe Martin profile. The overall appearance of the Bouguer anomaly field along the Combe Martin profile is closely similar to that along the Porlock profile, values falling northwards from a maximum of over 24 mgal on Exmoor to a minimum of o mgal in the southern half of the Bristol Channel and rising again to 12 mgal near the Welsh coast. Along the Combe Martin profile, as along the Porlock profile, low degree trend surfaces fail to reproduce the Bouguer anomaly field beyond the limits of the Bristol Channel low and the Exmoor gradient. Hence again the first degree surface was adjusted, by a maximum of 3 mgal, to yield a simple regional background ofo-29 mgal km -x defining an entirely negative residual anomaly. The width of the residual anomaly is nearly 4 ° km and its amplitude is --x 8 mgal. The greater amplitude of the residual anomaly

mgal 28

24 Somerset. elomo~on ~ co/stline co/stli~e ,,, ,: 20

I ,,! N ~ ~ ',~ / F'Irst degree I ~ ~..~l "~. trend surface I

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I I I I

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--4

--8 anomoly

-12 F1o. 5. Bouguer anomaly profile along longitude line 3°3o'W (Porlock profile).

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along the Combe Martin profile is consistent with the overall form of the residual anomaly shown in Fig. 4. From the known geology along the Porlock profile it can be deduced that the negative anomaly is due mainly to Palaeozoic structure. The Jurassic sequence cropping out on the sea bed in this part of the Channel extends no higher than the base of the Pliensbachian. Taking into account the known thicknesses of Keuper and Rhaetic successions in west Somerset and the Vale of Glamorgan and the outcrop width of Liassic zones on the local sea floor, it is concluded that the total thickness of the locally preserved Mesozoic succession cannot much exceed 25o m. On the assumption of a density contrast of o.x 5 g cm -3 with

toga!

s CM~.E N

20 \ Selectedoo'

\ \ / / obse,',,eaoooo,,

4 ~ / E xmoor gradient Bristol Channel low ,o ~ 30.m 0 * . • , I , , ' I I I I ' ' I , , i n ~ I t ! i ~ I , ~ n

0,vo0 \ / -12_1, coastline

-2o FIG. 6. Bouguer anomaly profile along longitude line 4°oo'W (Combe Martin profile).

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underlying normal Upper Palaeozoic strata, the Mesozoic rocks would give rise to a maximum effect of about 2 mgal along the axis of the Bristol Channel syncline, 8 km off the Somerset coast. At first sight the negative residual anomaly along the Combe Martin profile, centred approximately over the axis of the Bristol Channel syncline, may appear to be due entirely to the thick Mesozoic succession preserved locally in the syncline. Along this profile Kimmeridgian strata are preserved and the maximum Mesozoic thickness is probably about i6oo m. However, since the Exmoor gradient again lies mainly over Upper Palaeozoic rocks and extends 15 km south of the Palaeozoic to Mesozoic boundary, only a part of the negative anomaly can in fact be attributed to the Mesozoic succession. Calculations of the gravity effect of a wide range of models of the Bristol Channel syncline indicate that its anomaly must be less than one milligal at a remove of only i km. The Mesozoic rocks therefore cannot explain the main part of the Exmoor gradient. It must be concluded that the negative anomaly is composite, being due partly to the Bristol Channel syncline but partly to underlying Palaeozoic structure similarly to the situation along the Porlock profile. The possible geological explanations advanced by Bott et al. (I958) to account for the Exmoor gradient were: (I) an unexposed granite, (2) low-density Devonian rocks in normal stratigraphic sequence with, and underlying, the exposed succession of Exmoor, (3) low-density pre-Devonian rocks and (4) low-density Upper Palaeozoic rocks tectonically overridden by the Devonian rocks of Exmoor. These also represent the main possibilities for explaining the entire residual anomaly shown in Fig. 4. The linear nature of the anomaly and its wide flanking zones of fairly uniform gravity gradients count against a granite mass (as does Occam's razor). The close parallelism between the anomaly contours and the geological strike over a distance of more than 5 ° km suggests a causal sedimentary rock unit whose disposition is closely related to the surface geological structure. Thus the main possibilities are (I) Lower Devonian or underlying rocks simply involved in the surface folding, and (2) Upper Palaeozoic rocks in a concealed structure of similar trend beneath the Cannington thrust. Regarding the first possibility, it is easy to show that a continuous low density layer in any reasonable fold structure cannot reproduce the observed anomaly. However, Donovan (I97i) suggested that the Exmoor gradient may result largely from the rapid wedging out southwards, due to facies change, of sandstone units in the Devonian succession. In the following discussion the thrust hypothesis, favoured by Bott et al. (I958) is first re-examined in the light of the new gravity data and the alternative explanation in terms of facies change is then considered. It is shown that both hypotheses can account for the gravity anomaly and if a choice is to be made it must be based on regional geological considerations. Interpretations were carried out by computer using two programmes. The first calculates the gravity effect of a two dimensional structure by approximating its shape to an n-sided polygon (Talwani et al. I959; Grant & West I965); the second is an automatic programme similar to that described by Bott (196o) which calculates the gravity effect of a prismatic model built below a specified reference surface and, by an iterative process, adjusts the model shape until it accounts

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satisfactorily for the observed anomaly. The density of normal Upper Palaeozoic rocks was assumed to be 2.65 g cm -3 and models were constructed for hypothetical rock bodies o.Io and o.i 5 g cm -3 less dense. Corrections for the effect of Mesozoic rocks were based on an assumed average density of 2.5 ° gcm -3. Re-examination of the thrust hypothesis. The effect of applying a correction for the steep regional gradient, of which Bott et al. (I958) had no knowledge, is to reduce very significantly the amplitude of the residual anomaly and the steepness of the Exmoor gradient which forms the southern flank of the anomaly. Thus the above workers explained the entire drop of 22 mgal across Exmoor (from which they inferred a further reduction out into the Channel giving a total anomaly of about 3o mgal) in terms of concealed Upper Palaeozoic structure. After correction for the regional field the amplitude is reduced to IO regal in the case of the Porlock profile. The position of subcrop of the Cannington thrust under the Bristol Channel is very uncertain. Bott et al. (I 958) assumed that it ran close to the coastline as far west as Porlock, but they took no account of the possible effect of the Cothelstone fault 1 of the Q uantock Hills, the projection of which would cross the line of subcrop of the thrust in the vicinity of Watchet. It is clear from the gravity map that the Cothelstone fault does extend into the Channel and, as Owen (1971) indicated, it probably defines the linear coastline of Glamorgan from Nash Point to Swansea. The perfect alignment of this coastline with the southwest margin of the Q.uantock Hills suggests that, if there is an intervening major thrust, or set of thrusts, movements along the Cothelstone fault post date the main thrust movement. The actual displacement across the Cothelstone fault can only be deduced by indirect and imprecise means. Webby (i966) deduced a I O km displacement of the boundary between the Morte Slates and Ilfracombe Beds across the fault by extrapolating the course of the boundary eastwards from the Brendon Hills. By assuming that the ratio of horizontal to vertical movement was the same as for the Timberscombe fault of the Brendon Hills (Webby I965), he estimated a vertical downthrow of 2 km to the southwest and a dextral movement of 5 km for the Cothelstone fault. However, a simple dextral displacement of I O km would also explain the described effects of the fault. A downthrow of 2 km on the Cothelstone fault would displace the subcrop of a 5 ° dipping thrust by about 2o km northwards. In the interpretation of the Porlock gravity profile movement along the Cothelstone fault is assumed to have been mainly dextral and the subcrop of the thrust is accordingly moved about IO km northwards from the position suggested by Bott et al. Fortunately, the actual position of the thrust does not much affect the outcome of the gravity interpretation and influences only slightly the calculated thickness of underlying structures. Models capable of explaining the Porlock residual anomaly (Fig. 5) are shown in Fig. 7 and indicate an underlying structural basin filled with rocks of low density. The illustrated models incorporate a 5 ° dipping thrust but a I o ° thrust also

1 The much smaller displacement across the nearby Timberscombe fault will be ignored.

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produces satisfactory solutions, with slightly greater indicated thicknesses of underlying low density strata. The maximum thickness of low density Palaeozoic rocks is indicated to underlie the coastal area, reaching 2200 m for a density contrast of o.I 5 g cm -3 or 36oo m for a density contrast of o.Io g cm -3. These compare with values of about 5ooo m and 6ooo m computed respectively by Bott & Scott (i964) and Bott et al. (I958) using similar density contrasts. Mini- mum northerly transport along the thrust is indicated to be about 25 km. Along the Combe Martin profile the Mesozoic succession attains a thickness of about z6oo m and its gravity effect is therefore much larger than along the Porlock profile. Seismic profiling data have yielded the average dip of the nor- them and southern limbs of the Mesozoic fold, together with the position of the main fold axis and the position of the Mesozoic to Palaeozoic boundary off the north Devon coast. Hence a cross-section through the fold can be constructed with reasonable accuracy and the main uncertainty in computing the Mesozoic effect relates to the validity of its assumed average density of 2"5o g cm -3. Models accounting for the Combe Martin anomaly (Fig. 6) in terms of Meso- zoic and low density Upper Palaeozoic rocks are shown in Fig. 8 and indicate the existence of an underlying major basin as along the Porlock profile. Both models leave a small residual anomaly over the Bristol Channel syncline (Fig. 8) which is attributed to the existence of post Liassic strata with a density lower than

S WEST SOMERSET COAST BRISTOL CHANNEL N

mgal iJ +1 • • • I •

• i • . • • I I • • • - - - - -I " " " residual an•mary

0 5 10 15 20 25 5" . ..,... ; :'-.'.".':="./:!,~::..;..l.. • . -.,.- • ,'.':.,:':' :,'-,'". 't ".* "." "" " 1 • .. ~., .;.. ;.. "- "" ." . .',;'." • ;" " • ,, • ..;-', "..--.-.,.;, .=. • .. • , • , . 2 -".'::" ;."":--" ".;'.:.'.'~.:.-;.']:" : "'

43~L .... ~:'";"'" .... "" "" "" " .... " "• ": "' : "' "" "" : ;'"" :'" ; ;)''";'" F'-/ " O -3

Km .r c,, -0.15 gcm

toga! +I i

-1 L residual anomaly

0 5 10 15 20 25 Krn

- i:i.-}:i/': i~::i ": ~: i~ i-/:.:'. ~?.~>i ~::~. ~'. :: .....

~:~--0"10 fl crn -3 5L Km

FIo. 7. Interpretation of Porlock anomaly in terms of an overthrust structural basin containing low density Upper Palaeozoic rocks. The effect of the thin Mesozoic cover in the Bristol Channel was removed prior to the interpretation. The residual anomaly in this and later figures is the calculated anomaly minus the observed anomaly.

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2-50 g cm -3. The position of subcrop of the thrust was inferred to lie under the southern limb of the Bristol Channel syncline. Strictly, the thrust plane should be affected by the post Kimmeridgian folding but this complication may be ignored in the present interpretation without serious error. Again, a 5 ° dipping thrust is assumed, but thrusts with steeper dips also produce satisfactory solutions• The maximum thicknesses of low density Upper Palaeozoic rocks are preserved under the Bristol Channel syncline (Fig. 8) and are calculated to be 28oo m and 47oo m for the two model densities. Minimum transport along the oversimplified thrust shown in Fig. 8 is about z3 km. If the thrust is in fact folded under the Bristol Channel syncline a significantly greater amount of transport, comparable to that along the Porlock profile, is indicated. From the Bouguer anomaly map of Fig. z2 it is clear that if the Exmoor gradient is caused by the southern flank of an underlying structural basin, the basin most likely extends to the wr~v at least as far as the Sticklepath-Lustleigh fault line. To the east, the residual anomaly in the Channel attenuates towards the Cothelstone fault (Fig. 4) but only because of the diminishing contribution of Mesozoic rocks to the total anomaly. A strong northerly gradient is found over the Q uantock Hills and as far east as Bridgwater (Falcon, in Cook & Thirlaway z952 ) suggesting that the Upper Palaeozoic structural basin continues in that direction.

S DEVON COAST B R I S T O L C H A N N E L ingot i • • el [ • • • • • • • • • • • f , " • . ,l •

residual anomaly

O .5 10 15 20 25 30 km . I i i 5 i I

-~c = -0-15 g cm -3

Sfkm

r¢~l Q 4,1

-I L residuol anomaly

? 5 10 15 20 215 30 km 1 , 1 5 e i i

•~¢= -0.10 g crn -3

F1o. 8. Interpretation of Combe Martin anomaly in terms of Mesozoic strata in the Bristol Channel syncline and underlying Upper Palaeozoic rocks of low density.

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Facies change as a possible explanation of the Bristol Channel anomaly. If the main anomaly is to be explained by low-density rocks in the Devonian succession involved in the surface structure, it is necessary to postulate that the regional Hercynian dip to the south (which characterises Exmoor) persists under the Bristol Channel although later refolding must, of course, occur under the Bristol Channel syncline. A thick Devonian sandstone unit subcropping in the Channel and wedging out southwards can then reproduce the observed anomaly. In the model presented to explain the Porlock residual anomaly (Fig. 9) no attempt has been made to incorporate the effect of the major anticline shown trending ~s~. from Lynton by Edmonds et al. (I969, Fig. IO) to connect with the Courtway anticline of the Quantock Hills (Webby I966); there is too little documented evidence on which to base a structural section. Undoubtedly, the model shown in Fig. 9 is oversimple, but it indicates that the amplitude and width of the observed anomaly can be reproduced approximately by the effect of a thick sandstone unit wedging out at depth to the south. As would be expected, there is a close geometrical similarity between the overthrust basin of Fig. 7 and the sandstone wedge of Fig. 9. In the instance illustrated, the sandstone has a density of 2-5o g cm -3 and a maximum thickness of 2ooo m. If the 2 mgal depression in the southern flank of the Combe Martin profile (Fig. 6) is correctly interpreted as the gravity effect of the Hangman Grits, it is clear that the hypothetical sandstone lower in the succession must be much thicker and, or, exhibit a lower density in order to generate the main negative anomaly. On an earlier view of Devonian stratigraphy (e.g. Evans I922) the hypothetical sandstone could have been equated with the Foreland Grits which crop out in the vicinity of Foreland Point and were formerly regarded as the lowest member (base not seen) of the exposed Devonian succession• Now that the Foreland Grits are regarded as being a separate, structurally-controlled outcrop of the Hangman Grits (Webby i965) , the lowest member of the exposed Devonian succession is taken to be the Lynton Beds and any major sandstone unit must underlie these beds. The geological implications of the above interpretations are discussed in section 4.

c- ..D WEST SOMERSET COAST BRISTOL. CHANNEL N

rngol l +lj - . !

e • ..... • • II • --1 L residual onomoly

0 5 10 15 20 25 Km 1 I .... L ...... } ...... :-..: ~ .,. :./-~.:. ,.....: ~. :,'.,;...~:~'::.": :.:.; , i. :.y..: .; !-..~.."/"..'.q'..''.'.".!..'...... , ~.;....

-3 Thin cover of "~c == -O.15 gcm MeSOZOiC

,4 Krn

F 1G. 9. Interpertation of Por]ock anomaly in terms of a southerly dipping sandstone wedge.

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(E) THE BOUGUER ANOMALY FIELD SOUTH AND WEST OF GOWER The northern half of the Bristol Channel off Gower and Swansea Bay is largely covered by an extensive layer of recent sediments and we have no direct knowledge of bedrock geology except close in to the Gower coast. A (smoothed) residual map of this part of the Channel was produced by expressing the Bouguer anomalies of Fig. 12 against a second degree trend surface computed using all the gravity data. The map (Fig. Io) reveals a belt of positive residual anomaly, in places nearly 20 km wide, passing south of Caldy Island across Gower and the sea area to the south and extending eastwards towards the Vale of Glamorgan. The anomaly axis trends ESE, parallel to the axis of the Bristol Channel low to the south (Fig. 4), and peak values of +6 mgal are attained south of Gower from where the anomaly attenuates eastwards. The close association of this zone of positive residuals with Cower and the Vale of Glamorgan suggests that it delineates off-shore a wide belt of folded Carboniferous Limestone and Upper Old Red Sandstone existing near to surface. Seismic profiling by the authors reveals Mesozoic strata in many parts of the area, but these are presumed to form only a thin, intermittent cover to the folded Palaeozoic rocks, as on the Vale of Glamorgan (George i97o ). No major Mesozoic basins have been detected in this part of the Channel. A seismic refraction line extending west from the Scarweather Lightship (Fig. i o) close to the axis of the residual anomaly suggests that Old Red Sand- stone directly underlies about xoo m of Mesozoic strata (Mr D. G. James, pers. comm.). Therefore the gravity axis probably coincides with a major anticline, with a core of Old Red Sandstone, lying immediately south of the known folds of Gower and the Vale of Glamorgan. The main source of gravity anomaly is probably the anticlinal form of the underlying Lower Palaeozoic or Precambrian rocks. It is suggested therefore that the belt of positive residual anomaly overlies the central part of an anficlinorial structure which covers much of the northern half of the Channel and is thus on the same scale as the South Wales coal basin to the north and, perhaps significantly, the hypothetical Upper Palaeozoic basin concealed under the Bristol Channel syncline to the south (Figs. 7 and 8). The general disposition of residual anomalies in the Bristol Channel and Gla- morgan is strikingly similar to the Bouguer anomalies of the Wells and Cheddar (28o) Sheet (Brooks i965, Fig. I6). In that area a belt of positive anomalies several miles wide overlies the Mendip Hills, but peak values of about +4 mgal lie well south of the exposed periclines in an area of partial Mesozoic cover under which additional Hercynian folds are known to exist. To the north, Bouguer anomalies become negative in the Radstock coal basin and to the south they become negative over the thick Mesozoic succession of the central Somerset basin. A difference between the two areas is that although a concealed Upper Palaeozoic basin can be inferred to underlie the Bristol Channel syncline, Brooks (I965, p. i55 ) ruled out an analogous structure under the central Somerset basin.

(F) THE BRIDOWATER BAY HIGH This is interpreted tentatively as a continuation of the belt of positive anomaly overlying and south of the Mendip Hills (Brooks 1965) from which, however, it is

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°,~

~oa

/ O"

\ I 0 i /

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separated by a saddle in the gravity field. As such, the anomaly would connect westwards, across the projection of the Cothelstone fault, with the belt of positive anomaly south of Gower (Fig. Io). The disposition of the Bridgwater Bay high is consistent with the above suggested dextral displacement of Io km across the fault. Seismic profiling in Bridgwater Bay indicates the existence of gently folded strata of assumed Liassic age under a thin but continuous layer of sediment. On the above interpretation of the gravity high, the Liassic strata must be succeeded at shallow depth by Old Red Sandstone or older rocks in the core of the major anticlinorium postulated to extend from Pembrokeshire to the Mendips.

(G) THE REGIONAL FIELD The first degree trend surface computed by omitting data from the Lundy area indicates a 45 mgal rise in the regional field across the survey area, from --5 mgal in the mouth of the Severn to +4 ° southwest of Lundy, at a horizontal gradient of 0"38 mgal km -x. This southwesterly rise appears to be related to a circular region of high gravity (greater than +25 mgal) shown by Day and Williams (197o) to overlie the southern half of the outer Bristol Channel, southwest England (except for the Cornubian granite chain) and the northwest part of the English Channel. The northwestern part of this regional anomaly shows up well in the Bouguer anomaly map of the outer part of the Bristol Channel of Davey (197o) . Possible explanations for this region of high gravity include local crustal thinning or a local crust of above average density. For example, the gravity gradient across the Bristol Channel area could, in isolation, be explained by a I o regional dip on the base of the crust or by a horizontal change of 0.02 gcm -3 per IOO km in the average density of a crust 35 km thick. However, the fact that the regional gradient is markedly different in adjacent areas of Wales, southwest England and the Celtic Sea counts against such a deep origin. Crustal seismic experiments in southwest England carried out by Holder & Bott (I968) yielded insufficiently precise results to give any insight into the significance of the regional gravity anomaly. 4-Discussion The gravity map leads to the solution of many structural problems in the Bristol Channel area but leaves some important issues unresolved. Of these, perhaps the most important is the significance of the negative anomaly overlying the Bristol Channel syncline and the belt of Devonian strata to the south. Whichever geological model is preferred to account for the anomaly, namely an overthrust structural basin (Figs. 7 and 8) or a dipping sandstone wedge (Fig. 9), a major fault must exist somewhere under the Channel. With an overthrust basin, it is the thrust itself with an indicated minimum transport of 25 km; with a wedge, a Lower Devonian or earlier sandstone is required to crop out in the Channel adjacent to the known Carboniferous Limestone of the Vale of Glamorgan and, further west, the Carboniferous Limestone on the southern flank of the postulated anticlinorium. This juxtaposition would require a fault with a throw of several thousand metres, for which there is no geological evidence in the region and no apparent fault

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anomaly on the gravity map. By contrast, if the basin model is accepted and its contained rocks are assumed to be of Upper Carboniferous age, the Hercynian structure of the autochthon of the Bristol Channel is viewed as comprising, mainly, a sequence of Old Red Sandstone and Carboniferous rocks (of South Wales facies) with a regional dip to the south as shown in Fig. I I. The basic simplicity of this regional structure inclines the authors strongly in its favour. Undoubtedly, any such structure will be complicated considerably by subsidiary folding and faulting. The postulated existence under the Channel of a basin filled with Upper Carboniferous strata has regional structural, stratigraphic and palaeogeographic implications which cannot be fully discussed here. One important question relates to the likely density of the Upper Carboniferous sequence, for this determines its indicated thickness. The Upper Carboniferous of the South Wales coalfield has a high density and if the postulated basin contains rocks of similar lithology the thicker models of Figs. 7 and 8, based on a density contrast of o.Io gcm -3 will be expected to simulate more closely the actual shape of the basin. Cook & Thirlaway (I952) concluded that in the Bristol and Somerset coalfield the density contrast between Coal Measures and earlier rocks must be at least o.i o gcm -s. On the assumption that the negative anomaly in the southern half of the Bristol Channel and the Exmoor area is due to an overthrust structural basin containing Upper Carboniferous strata, it is possible to attribute part or all of the regional gradient across the Channel to a northward thickening of low density sediments of pre-Carboniferous age. However, as discussed above, the entire regional gradient can equally well be accounted for by deeper, crustal effects. The minimum amount of movement along the Cannington thrust can be determined only very approximately from the gravity interpretation and the actual movement cannot be assessed without knowing the age of rocks preserved in the allochthon under the Channel. However, the overall movement is likely to exceed 25 km. Adjustment for tectonic shortening of this magnitude leaves a wide transitional zone in which major changes of facies, from continental Old Red Sandstone to marine Devonian and from Carboniferous Limestone to Culm Measures, can occur. On this view the Carboniferous Limestone of Cannington

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Park, being of the northerly facies, would be expected to underlie rather than overlie the major thrust, in spite of Webby's (1966) arguments. The importance of faulting in the Channel is indicated by the gravity anomalies associated with the Cothelstone and Sticklepath-Lustleigh lines. Much other faulting has probably gone unrecorded. For example, it is clear that many of the coastlines around the Channel are defined by post Mesozoic faulting (Owen 197 I) ; and complex outcrop patterns of Jurassic zones are indicative of important strike faults affecting the Bristol Channel syncline. These faults appear to have little effect on the gravity field. In summary, the structure of the Bristol Channel is interpreted as comprising in its northern half an Hercynian anticlinorium with a thin Mesozoic cover separating the South Wales coal basin to the north from a closely similar structural basin under the southern half of the Channel and adjacent land areas. The southern basin is concealed under a major Hercynian thrust and a thick overlying Mesozoic succession preserved in the Bristol Channel syncline.

ACKNO~VLEDGEMENTS. We thank most sincerely the Captains and crews of R.R.S. John Murray and research vessel Ocean Crest for their kindness and co-operation during the surveys. Mr R. Bradley (N.E.R.C.) was in charge of the LaCoste and Romberg meter and offered much other assistance. Mr D. G. James and Mr S. M. Newton kindly helped with the field work. We are most grateful to the Institute of Geological Sciences for making gravity data available for our use.

5. References AL-SADI, H. N. x967. A gravity investigation of the Pickwell Down Sandstone, north Devon. Geol. Mag. xo4, 63-72. ANDERSON, J. G. C. & OW~N, T. R. 1968. The structure of the British Isles. Pergamon Press. Boar, M. H. P. 196o. The use of rapid digital computing methods for direct gravity interpretation of sedimentary basins. Geophys. J. R. astr. Soc. 3, 63-7. & SCOTT, P. 1964 . Recent geophysical studies in south-west England. In Present views on some aspects of the geology of Cornwall and Devon. Blackford Ltd., Truro, Cornwall, PP. 25-44. , DAY, A. A. & MASSON-SMrrH, D. 1958. The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Phil. Trans. R. Soc. 25xA, 16x-9I. BRooKs, M. I965. Geophysical investigations. In Green, G. W. and WELCH, F. B. A. (Eds.) Geology of the country around Wells and Cheddar. Mem. Geol. Surv. G.B. I966. A study of density variations in New Red Sandstones from the English Midlands. Geol. Mag. xo3, 6t-9. CooK, A. H. & THIRLAWAY,H. I. S. I952. A gravimeter survey in the Bristol and Somerset coalfields. Q. Jl geol. Soc. Lond. xo7 (for 195I), 255-86. &Mum, Hv, T. 1952. Measurements of gravity in Ireland. Gravity survey of Ireland north of the line Sligo-Dundalk. Geophys. Mem. 2(4), Dublin Inst. Adv. Studies. CORD~LL, L. & HVND~RSON, R. G. 1968. Iterative three-dimensional solution of gravity anomaly data using a digital computer. Geophysics 33, 59 TM COmCV~LL, J. D. 197 x. Geophysics of the Bristol Channel area. Proe. geol. So¢. Lond. x664, 286--9. DAv~Y, F. J. x97o. Bouguer anomaly map of the North Celtic Sea and entrance to the Bristol Channel. Geophys. J. R. astr. Soc. 2"~, 277-82. DAY, G. A. & WILLIAMS, C. A. x97o. Gravity compilation in the N.E. Atlantic and interpretation of gravity in the Celtic Sea. Earth and Planet. Sci. Letters 8, 2o5-x3. DF.ARMAN, W. R. I963. Wrench-faulting in Cornwall and south Devon. Proc. Geol. Ass. 7'1, 265-87.

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DOLLAR, A. T. J. i941. The Lundy complex: its petrology and tectonics. Q. dl geol. Soc. Lond. 97, 38-77 • DONOVAN, D. T. I963. The geology of British Seas. University of Hull. , SAVAGE, R.. J. G., STRIDE, A. A. ~L STUBBS, A. 1~. 196I. Geology of the floor of the Bristol Channel. Nature, Lond. x89, 51-2. ~, LLOYD, A. J. & STRmE, A. H. I97I. Geology of the Bristol Channel. Proc. geol. Soc. Land. I664, 294-5. EDMONDS, E. A., McKa~owN, M. C. & WILLIAMS, M. I969. South-West England. Br. Reg. Geol. 3rd edn. EVANS, J. W. 1922. The geological structure of the country around Combe Martin, north Devon. Pro¢. Geol. Ass. 33, 2Ol-28. FASI-IA~, M. J. R. 1971. A gravity survey of the Bovey Tracey basin, Devon. Geol. Mag. xo8, I 19-3o. FOURMARIER, P. F. J. 1933, Observations sur l'estimation de l'importance du transport suivant le 'charriage du Condroz'. Ann. Soc. gdol. Belg. 56, 249-59- GEORGE, T. N. 197 o. South Wales. Br. Reg. Geol. 3rd edn. GRANT, F. S. & WEST, G. F. I965. Interpretation theory in applied geophysics. McGraw-Hill. HAMILTON, D. & BLUNDELL,D. J. I97I. Submarine geology of the approaches to the Bristol Channel. Proc. geol. Soc. Lond. 1664, 297-3oo. HOLDER, A. P. & BOTT, M. H. P. I968. Crustal Structure of Great Britain. A.R.P.A. Vela Uniform program. S.R.P.G. Final Scientific report. LACOSTE, L., CLARKSON,N. & HAMILTON,G. 1967. LaCoste and Romberg stabilized platform shipboard gravity meter. Geop,~ysics 32, 99-IO9. LLOYD, A.J. i963. Upper Jurassic rocks beneath the Bristol Channel. Nature, Lond. I98 , 375-6. McQuILLIN, R. & TUSON, J. 1963. Gravity measurements over the Rhum Tertiary Plutonic complex. Nature, Lond. 199 , I276-7 . MILLER, J. A. & FITCH, F. J. i962. Age of the Lundy Island granites. Nature, Land. x95 , 553-5. O'LEARY, M., LIPPERT, R. H. & SPITZ, O. W. I966. Fortran IV and map program for com- putation and plotting of trend surfaces for degrees I through 6. Computer Contribution 3, State Geological Survey Kansas. OWEN, T. R. x97I. The structural evolution of the Bristol Channel. Proc. geol. Soc. Land. I664b 289-94. PARASNlS, D. S. 1952. A study of rock densities in the English Midlands. Mon. Not. R. astr. Soc. (Geop.~ys. Suppl). 6, 252-71 . RAMSB~TTOM, W. H. C. I97o. Carboniferous faunas and palaeogeography of the south-west England region. Proc. Ussher Soc. % 144-57. REABmO, H. G. I965. Recent finds in the Upper Carboniferous of south-west England and their significance. Nature, Land. 2o8, 745-7. RICHEY, J. E. I932. Tertiary ring structures in Britain. Trans. geol. Soc. Glasgow 19, 42-I4O. SHEARMAN, D. J. I967. On Tertiary fault movements in north Devonshire. Proc. Geol. Ass. 78, 555-66. TALWANI, M., WORZEL, J. L. & LANDISMAN, M. 1959. Rapid gravity computations for two- dimensional bodies with application to the Mendocino submarine fracture zone. J. geophys. Res. 64, 49-59. THOMAS, M. D. 1968. Gravity surveys around the Mouth of the Severn. Unpublished Ph.D. thesis, University of Wales. & BI~OOKS, M. (1973 in press). The geological significance of a negative gravity anomaly in the South Wales coalfield. Geol. J. 8. USSHER, W. A. E. 189 I. On the probable nature and distribution of the Palaeozoic strata beneath the Secondary, etc., rocks of the Southern Counties, with special reference to the prospects of obtaining coal by boring south of the Mendips. Proc. Somersetsh. Archaeol. Nat. Hist. Soe. 36 (for I89O), 88-I36. WALLlS, F. S. 1924. The Avonian of Cannington Park, near Bridgwater, Somerset. Geol. Mag. 6x, 218--25.

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W~.BBY, B. D. x965. The stratigraphy and structure of the Devonian rocks in the Brendon Hills, west Somerset. Proc. Geol. Ass. 76, 39-6o. t966. The stratigraphy and structure of the Devonian rocks in the Quantock Hills, west Somerset. Pro¢. Geol. Ass. 76, (for I965), 32x-44. WELCH, F. B. A. x933. The geological structure of the Eastern Mendips. Q. Jl geol. Soc. Lond. 89, I4-52. WHrTTON, J. T., MYERS,J. O. & WATSON,I.j. I955. The geological results of measurements of gravity in east Carmarthenshire. Geofis. pura appl. 32, 43-53.

Received 2 t September t 971 ; revised manuscript received x3 January x972; read I5 March z972. Michael Brooks and Martin Scott Thompson, Department of Geology, University College, Swansea, Wales.

DISCUSSION M. H. P. BOTT congratulated the authors on their contribution to shelf geology and was pleased to note the general agreement with the earlier interpretations made by the speaker and his colleagues. The controversial point of the paper seemed to be the very steep linear gradient across Exmoor and the Bristol Channel assumed by the authors, which itself must be caused by a shallow structure. Another possibility is a much less steep gradient associated with thicker low density Carboniferous/O.R.S. sediments beneath the northern part of the Bristol Channel. Perhaps the true structure beneath Exmoor lay between the extremes suggested by the speaker and his colleagues (flat regional) and by the authors (steep regional).

Replying on behalf of the authors, DR BROOKS agreed that the linear regional gradient of about o. 4 mgal/km is likely to have a shallow origin. Although, as pointed out in the paper, the gradient could in isolation be regarded as being caused by a slight dip on the Moho, or by a slight progressive change of overall crustal density across the survey area, the markedly different regional fields in the adjacent areas of Wales, southwest England and the Celtic Sea count against such a deep origin. Nonetheless, with regard to the proposed thickness of Carboniferous sediments in the overthrust basin the authors consider that there is little room for man- oeuvre. It is argued in the paper that the likeliest cause of the Bristol Channel low (after correction for the effect of Mesozoics) is low density Upper Carboni- ferous rocks. On this view the background field used in the interpretation must be effectively zero over areas of Lower Carboniferous outcrop, as has been arranged in the interpretation presented. There is however a possibility, also mentioned in the paper, that a wedge of low density Old Red Sandstones thickens northwards under the Bristol Channel area and contributes to the regional fall of gravity in that direction. Thus, although the thickness of the Upper Carboniferous sequence

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appears to be well established, the total thickness of Upper Palaeozoic sediments may approach that given by Bott et al. (1958).

DR BULLERWELL asked whether the authors had considered the possibility of a causal relationship between the source of the regional field and the Lundy igneous centre.

DR BROOKS replied that this interesting possibility has not been considered, and its investigation would require a study of the gravity field over a wider region than the present survey area. However, in this context it is worth noting that most of the Tertiary centres, like the Lundy centre, lie in areas of high background anomaly. This could be explained if the intrusive complexes were associated with underlying crustal zones of extensive basic intrusion.

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