A New Geological Model to Explain the Gravity Gradient Across Exmoor, North Devon

A new geological model to explain the gravity gradient across Exmoor, north Devon

M. BROOKS, M. BAYERLY & D. J. LLEWELLYN

SUMMARY Recent long seismic lines in South Wales and plained by a simple geological model in which the Bristol Channel indicate a structural cul- a thick sequence ofrelatlvely low density Lower mination under the southern part of the Bristol Palaeozoic or late Precambrian rocks occupies Channel, where a layer with a seismic velocity the core of this culmination. The model casts of 6-I km/s approaches to about 2 km of the further doubt on the existence of a major thrust surface. It is shown that the gravity field across under Exmoor. Exmoor and the Bristol Channel can be ex-

I. Introduction SEVERAL LONG SEISMIC LINES in the Bristol Channel area, full details of which will be presented in later papers, give evidence of a basal layer of high velocity, in the range from 6. I-6. 3 km/s, which is interpreted as being of Lower Palaeozoic or Precambrian age. This layer, which almost certainly does not represent the same geological formation under all lines, lies at shallow depth under the western part of the South Wales Coalfield, deepens southwards into the northern part of the Bristol Channel and rises rapidly towards the north Devon coast. Figure I illustrates reduced time-distance curves for two of the long seismic lines D and F, whose locations are shown in Fig. 2. On each line, a series of offshore shots was fired into a fixed array of land recording stations. Although the lines are not reversed, the configuration of shots and recorders enables an estimate of refractor dip to be made and it is concluded that the refracting interfaces are essentially horizontal under both lines. Horizontal layer interpretations of the time-distance data are shown in Fig. I and indicate that the high velocity basal refractor lies at a depth of 5 km in mid-Channel but shallows to 2 km off the north Devon coast. The Bouguer anomaly field across north Devon and the Bristol Channel is reassessed in the light of these findings, with particular reference to the postulated Exmoor/Cannington thrust under Devon and west Somerset.

2. The gravity field over north Devon and the Bristol Channel

Bouguer anomaly values decrease northwards across Exmoor and west Somerset. Falcon (in discussion of Cook & Thirlaway I952 ) suggested that the gravity gradient across the Quantock Hills of west Somerset might be associated with an underlying thrust, and Bott et al. (i 95 8) attributed the gradient to Upper Palaeo-

aTl geol. Soc. Lond. vol. I33, x977, pp. 385-393, 2 figs. Printed in Great Britain.

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LINE D J

• t

0"5

I I I I 110 20 30 40 50 610krn

W Vale of Glamorgan 4.60 + 0.07 km/s

5.60-+0.04 km/$

6"!2+-0-02 km/s 6 km

[ LINE F J

I I I I I 10 20 30 40 50 km

Exmoor W" 5-43 _+0-03 km/$

5-65--.0.04 km/s

6.17± 0"01 km/s

FzG. t. Reduced time-distance curves and interpretation models for long seismic lines D and F (sea level datum).

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zoic rocks of low density extending under the Channel and partially overthrust by the Devonian sequence of north Devon. The Bouguer anomaly map of the Bristol Channel (Brooks & Thompson 1973, fig. 12) shows that the Exmoor gradient extends WNW out into the Channel and is succeeded northwards by a linear negative anomaly overlying the thick Mesozoic sequence preserved in the Bristol Channel syncline (Lloyd et al. 1973). Anomaly values are much higher in Devon than in Cower and the Vale of Glamorgan. Thus before interpreting local anomalies Brooks & Thompson corrected for a regional gradient, with the effect of markedly reducing the Exmoor gradient and rep- resenting it as the southern flank of a broad negative anomaly covering a large area of the Channel, north Devon and west Somerset. This negative anomaly they attributed party to Mesozoic strata preserved in the Bristol Channel syncline and partly to an underlying low density rock unit extending beneath Exmoor. The essential feature of the model was that the concealed low density unit thinned out to north and south and the geological interpretation preferred by Brooks & Thompson invoked Upper Carboniferous sediments in a structural basin partially overthrust by the Devonian sequence. However, they pointed out, as had Bott & Scott (x 964) in a review of the earlier interpretation of the Exmoor gradient, that the anomaly could be explained without recourse to a major thrust: the concealed low density unit could be viewed as a thick sandstone sequence within or at the base of the Devonian succession and wedging out southwards.

3. Geological discussion of the thrust hypothesis Falcon (op. dr.) based his suggestion of a major thrust under the Q uantock Hills 'mainly on the evidence of the Cannington inlier' (northwest of Bridgwater). The proposal for a thrust in the Cannington area can be traced back to Ussher (I89i) who wrote: 'we have not a shred of evidence in favour of unconformity between the Cannington Park Carboniferous Limestone and the Devonian (Rodway Beds of the Rodway inlier to the south). Therefore the fault (separating them) must be of sufficient magnitude to cut out a considerable part of the Devonian series. As it is not exposed we can only conjecture as to the nature of this dislocation, which might be a thrust plane'. The diffidence of this suggestion has tended to be over- looked in all subsequent debate. Recent investigations by the Institute of Geological Sciences have shown that the Rodway Beds are of Namurian age and are underlain by Carboniferous Lime- stone (Whittaker I975). Whittaker attributed the tectonic juxtaposition of high Namurian and Carboniferous Limestone to lag faulting and normal faulting along the line originally suggested by Ussher to be one of thrusting. This new finding leaves Namurian strata closely adjacent to Middle Devonian outcrops in the Halseycross Farm inlier southwest of Cannington Park, suggesting an intervening major fault with a northerly downthrow of perhaps several thousand metres. Whittaker suggested that this fault could be the near-surface trace of the Exmoor/ Cannington thrust but, clearly, the evidence of the Cannington Park area should not be used as the main justification for incorporating a major thrust into structural models of Exmoor and the Bristol Channel.

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A case for major Hercynian thrusting in the Bristol Channel area has also been made by comparison with tectonic events in other parts of the Hercynian belt of western Europe (e.g. Van Waterschoot van der Gracht I938 ), but such regional considerations would tend to locate the zone of thrusting north of the Channel (e.g. Pembrokeshire) rather than south of it (Matthews I974). It must be con- eluded that the geological case for a major thrust under Exmoor is circumstantial and less than convincing. Though it has proved very convenient in helping to explain the local gravity field, the thrust remains an hypothetical structure. Find- ings from the long refraction lines provide new insight into possible sources of gravity anomaly, and reinterpretation of the gravity data shows that a simple structural model, free of major thrusting, can explain all the features of the local field.

4- Reinterpretation of a Bouguer anomaly profile across north Devon and the Bristol Channel The gravity profile (Fig. 2) reinterpreted here is that along longitude line 4°oo'W originally interpreted by Brooks & Thompson (I973, fig. 6; Combe Martin profile). Gravity anomalies were calculated for two-dimensional models in which the cross-sectional shape of each geological unit was expressed in polygonal form (Talwani et al. 1959). Continuous rock units were extended for a considerable distance beyond the ends of the profile to avoid truncation errors. In the vicinity of the Combe Martin profile, the southern part of the Channel is occupied by a thick Mesozoic sequence preserved in the Bristol Channel syncline (Lloyd et al. I973, Evans I973, Brooks & Thompson I973 ). The Mesozoic geology has been established on the basis of extensive bottom sampling, CSP and sonar surveys and seismic refraction (Brooks & James I975) surveys. Estimates of the thickness of the local Mesozoic sequence derived from the CSP and refraction surveys are not in perfect agreement, and the present interpretation utilizes the higher estimates based on CSP interpretation. An overall thickness of about 24oo m is indicated. From the point of view of densities, this sequence is sub- divided into two units: the lower part of the Liassic sequence (alternating lime- stones and shales) and the underlying, thin Triassic sequence are given an average density of 2.5 ° g/era3; the higher Jurassic sequence of clays and sands is given an average density of 2"45 g/cm 8. The regional structure of north Devon and the thicknesses of individual Devonian and Culm formations are taken mainly from Edmonds et al. (1975). On this view the main sequence dips south at about 3 °0 in the southern limb of a major anticline with Lynton Beds occupying its core. The axis of the anticline extends WNW from the vicinity of the Brendon Hills of Somerset towards Lynton and out into the Bristol Channel (op. dt., fig. IO). The density measurement of Bott et al. (I958) indicate a density contrast of o-I 5 g/cm s between the argillaceous and arenaceous parts of the Devonian sequence. This distinction could be freely interpreted to imply a similar density contrast between, on the one hand, the Ilfracombe Beds and higher units and, on

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the other, the Hangman Grits (including Foreland Grits), Lynton Beds and lower, concealed parts of the sequence. AI-Sadi (x 967) listed densities for various Devon- ian and Culm formations, and his average formation density of Ilfracombe Beds and later units is o. 12 g/cm 3 higher than that of the underlying sequence. However, further density measurements by the authors suggest that the average densities of the two parts of the sequence may be closer: 2.73 g/cm 3 for the upper part (three measurements) and 2"67 g/cm 8 for the lower part (six measurements). Quite possibly, this density contrast varies across the region. The northern half of the Channel is assumed to be floored locally by Old Red Sandstone beneath a thin, discontinuous veneer of Mesozoic strata (Brooks & James x975). This has been ascribed an average density of 2"62 g/cm 3, which is the value calculated by Thomas & Brooks (1973) for the Old Red Sandstones of South Wales. There may alternatively be a considerable thickness of Carboniferous Limestone preserved south of Gower, with a likely density of 2.7 o g/cm 3 (Thomas & Brooks I973). However, in calculating the overall gravity anomaly, Carboni- ferous Limestone near to surface can be traded off against Lower Palaeozoic at depth without significantly altering the resultant geological section, and the limestone layer has therefore been ignored in the interpretation presented. The lateral transition of Old Red Sandstone into marine Devonian is schematized by a tran- sitional belt of intermediate density (Fig. 2). The depth to high velocity Lower Palaeozoic or Precambrian under the long seismic lines in the central and northern Bristol Channel implies the existence of a considerable thickness of overlying Lower Palaeozoic strata of lower velocity. These might correlate with layer 2 on line D (Fig. i). There is no direct geological evidence of their nature, and in the gravity interpretation they have been arbit- rarily ascribed a density of 2.7o g/cm 3. The velocity of 6. I-6"3 km/s exhibited by the underlying layer may be used in conjunction with the Nafe-Drake curve (Nafe & Drake I963) to suggest a density of 2.73 g/cm 3 and this density was used for initial investigation of the gravity anomaly. The attribution of this density to the basal layer requires the Exmoor gradient to be explained by the relatively low density of the lower and concealed parts of the Devonian sequence. Even using the maximum estimated density contrast of o-I 5 g/cm 3 (see above) it was not possible to match either the maximum amplitude or the form of the gravity field across north Devon and the southern part of the Bristol Channel. Consequently, the possibility was explored that the basal layer of the gravity model exhibits a relatively low density of 2-65 g/cm 3. For this phase of interpretation, the intra-Devonian density contrast was reduced to o.o 3 g/cm ~. It then proved simple to explain the Exmoor gradient in terms of a structural culmination under north Devon and a simple southerly dipping sequence of Devonian and Culm (Fig. 2). The lower level of gravity field in the southern part of the Channel, under which the depth to the basal low density layer increases northwards in line with the new seismic evidence, is attributed to the lower average density of the overlying Palaeozoic sequence in the latter area. In this model the Exmoor gradient itself results from the southerly fall, to depths of several kilometres, of a very thick, relatively low density unit lying beneath the outcropping Devonian sequence.

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A very similar geological model can be derived to explain the more pronounced gravity gradient across the Q.uantock Hills, the main differences being the require- ment of a slightly larger density contrast--o.o6 g/cm3--between the upper and lower parts of the Devonian sequence and a much thinner Mesozoic sequence (about i km) in the Bristol Channel immediately west of Bridgwater Bay. It is important to state that the conclusions of the present paper do not depend upon all the assumed rock densities being correct. The essential point of the new interpretation is that by making reasonable assumptions about the prevailing densities, a particular type of geological model emerges to explain the gravity field. The model would be changed in detail if one or more of the selected densities were substantially in error; but the overall structural implications would be unchanged

regal 24- ,..0,T A 20

South North o Ib 20 Jo k~ EXMOOR BRISTOL CHANNEL Coast S~,,~m,c B.Channe, syncli.ne~...Cfen!!azlo.~C:...... S!!ie'C ...-

Culm.IU&M.. Dev M.&L. Dev (2.~II" ' "":/:11111(2 45&2 50)~'Mes°z°lc.1.:.1...... , o o ° } oOR&. o o

FIo. 2. Reinterpretation of Bouguer anomaly profile along longitude line 4°oo'W (Combe Martin profile). Key to inset: SL = seismic line; G - gravity profile; CP = Cannington Park. Dots indicate computation points.

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so long as a density inversion existed below the base of the exposed Devonian sequence. 5. The nature of the basal seismic layer As stated above, the 6. i-6. 3 km/s seismic layer almost certainly represents different geological horizons under the various seismic lines along which it has been located. In particular, whereas the high velocity material under the western half of the South Wales coalfield is likely to comprise Precambrian igneous rocks similar to those cropping out nearby in Pembrokeshire, the featureless aeromagnetic field off the Devon coast precludes a similar interpretation of the shallow high velocity material under that area. For this reason, the high velocity layer shallowing southwards across the southern half of the Bristol Channel is assumed to comprise sedimentary or metasedimentary rocks of Lower Palaeozoic or late Precambrian age. Among sedimentary rock types, the high velocity of at least 6.1 km/s is best accounted for by a massive limestone or quartzite succession. However, such a succession need represent only a thin unit overlying a much thicker sequence of sediments of lower velocity and low density--perhaps sandstones and conglomer- atesnresponsible for the main gravity anomaly. Possible candidates for the high velocity layer include Lower Palaeozoic quartzites which, in southern Britain, are known in the Ordovician of south Cornwall (Sadler 1973) and the Lower Cam- brian of the English Midlands. In this connection it is very interesting to note that the Ludlovian Trichrfig Beds of the Llandeilo and Llandovery areas of South Wales may have been derived from the erosion of quartzites outcropping to the south, and that the overlying Downtonian Long Quarry Beds were probably derived from subsequently unroofed mica schists (Potter & Price 1965; Owen 1967). Quartz-mica schists could have a sufficiently low density to account for the main gravity anomaly if preserved thickly enough under the Bristol Channel, north Devon and west Somerset. Also relevant to the pre-Upper Palaeozoic geology of the Bristol Channel is the Lower Devonian Llanishen Conglomerate of the Cardiff district, which was probably derived from a nearby source to the south (Allen x975)- The composition of exotic clasts suggests that there were outcrops of volcanics, sandstones and quartzites (including Upper Llandovery quartzites) in the inner Bristol Channel area at that time. However, the Bristol Channel area was then presumably blanketed by a thick cover of later Devonian and Carboniferous deposits. These could have overlain a varied outcrop pattern of rocks ranging from Precambrian schists to Silurian sediments and volcanics. Renewed earth movements in the late Carboniferous led to the erosion of Coal Measures south of the South Wales coalfield, evidenced by southerly-derived sediments including intraformational conglomerates with coal pebbles in the Pennant Measures of the coalfield (Kelling I968 ). There is, however, no evidence that the Upper Palaeozoic cover in the Bristol Channel area was widely removed at that time, or subsequently, and during the Mesozoic the Channel received a further thick sedimentary cover, still largely preserved in major downfolds. Con- sequently, any thick sequences of Lower Palaeozoic or late Precambrian sediments

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or metasediments originally preserved beneath the Upper Palaeozoic cover of the Bristol Channel area are likely to be still largely preserved. It is hoped that further seismic investigations will throw light on the nature and geological significance of these sequences.

AG'Ir.NOWLEDGEMENTS.The authors are indebted to Mr T. R. Owen for discussion of the geological ideas presented in this paper. The marine seismic lines were supported by the Natural Environ- ment Research Council through the award of ship time, a research studentship (to DJL) and a research grant for the purchase of explosives.

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WHITTAKER,A. x975. Namurian strata near Cannington Park, Somerset. Geol. Mag. xx2, 325--6 (Correspondence).

Received t6 June t976; revised typescript received 2o November x976.

M. BRoom, M. BAYERLY & D.J. LL~W~LLVN, Department of Geology, University College, Swansea, Wales, Great Britain.

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