A New Geological Model to Explain the Gravity Gradient Across Exmoor, North Devon
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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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/4/385/4885496/gsjgs.133.4.0385.pdf by guest on 27 September 2021 386 M. Brooks, M. Bayerly & D. 07. Llewellyn LINE D J • t <3 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 km FzG. t. Reduced time-distance curves and interpretation models for long seismic lines D and F (sea level datum). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/4/385/4885496/gsjgs.133.4.0385.pdf by guest on 27 September 2021 Gravity gradient, Exmoor 387 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/133/4/385/4885496/gsjgs.133.4.0385.pdf by guest on 27 September 2021 388 M. Brooks, M. Bayerly & D. jT. Llewellyn 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 dis- tance 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.