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Seismic refraction surveys in the Channel 179

DONOVAN, D. T., SAVAGE, R. J. G., STRIDE, A. H. & STUBBS,A. R. i96I. Geology of the floor of the . Nature, Lond. i89, 5i-2. EDMONDS, E. A., McKEowN, M. S. & WILLIAMS, M. 1969. Southwest . Br. reg. Geol. EVANS, D. J. 1973. The stratigraphy of the central part of the Bristol Channel. Ph.D. thesis, Univ. (unpubl.) FENNING, P. J. & FRESHNEY, E. C. I968. Preliminary results of a borehole at Petrockstow, north Devon. Geol. Mag. IO5, I88-9o. FLETCHER, B. N. 1975. A new Tertiary basin east of Island. Jl geol. Soe. Lond. I3l , 223- 5. HILL, M. N. I963. Single-ship seismic refraction shooting. In Hill, M. N. (ed.) The sea. 3. London. LLOYD, A. J. I963. Upper Jurassic rocks beneath the Bristol Channel. Nature, Lond. x98, 375-6. LLOYD, A. J. SAVAGE, R. J. G., STRIDE, A. H. & DONOVAN, D. T. 1973. The geology of the Bristol Channel floor. Phil. Trans. R. Soe. 271A, 595-626. MILLER, J. A. & FITCH, F. J. I962. Age of the Lundy Island granites. Nature, Lond. I95 , 553-5- OWEN, T. R. I971. The structural evolution of the Bristol Channel. Proe. geol. Soc. Lond. I664, 289-94. PALMER, C. P. 1972. The Lower Lias (Lower Jurassic) between Watcher and Lilstock in north (). Newsl. Stratigraphy 2, 1-3o. TRUEMAN, A. E. I922. The Liassic rocks of . Proc. Geol. Ass. 33, 245-84 • --I93O. The Lower Lias (Bucklandi Zone) of , Glamorgan. Proc. Geol. Ass. 4I, I48-59. WOBBER, F. J. I965. Sedimentology of the Lias (Lower Jurassic) of South Wales. Jl sedim. Petrol. 35, 683-703.

Manuscript received 20 March 1974; revised typescript received 22 May 1974; read 3o October 1974.

MICHAEL BROOKS, Department of Geology & Oceanography, University College of , Wales.

DAVID GARETH .lAMES, Gevers Deynootweg 186A, Scheveningen, Den Haag, 2016, The Netherlands.

DISCUSSION THE PRESIDENT thanked Dr Brooks for this further contribution to data bearing on the structure of the Bristol Channel basin. He expressed surprise at the similarity of energy propagation velocities quoted for the basal Lias ( with at least equal parts of shale) and for the , presumed to be developed as a massive carbonate, and asked if there was some local ex- planation.

DR BROOKS replied that although the lower part of the lower Lias of the contains about equal parts of limestone and shale, there are sequences (notably in the bucklandi Zone) in which the proportion of limestone approaches 80% of the total thickness and within which individual massive limestone bands may be up to a few metres thick. Recent land refraction investigations west of Aberthaw produced a lower Lias velocity of 4.05 4-o'15 km/s (provisional value). This value lies close to assumed Carboniferous Limestone velocities encountered in the present surveys and draws attention to the problem of assessing the geological significance of refractor velocities in the range from 4"00 km/s to 4.5 ° km/s.

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D. R. TAPPIN commented on I.G.S. Borehole 72/6o (Brooks & James fig. 5) from which a Tremadocian age has been obtained. The core is 2'5 m in length and is vertically cleaved. It was recovered in a shattered condition beneath 5 m of sand and gravel and 3 m of stiff, brown, stoneless clay. The rock is dark grey, thinly laminated and highly cleaved. Microscopic examination (K. S. Siddiqui personal communication) shows that it has a medium to coarse grade (0.02-0.05 mm) silty texture, and has undergone low grade metamorphism. Professor C. Downie (University of Sheffield equated a sample, palynologically, with the Brachiopod Beds of the Shineton Shales. The Institute's work in the area includes Continuous Seismic Profiling (CSP) lines that run N-S and E-W, one of the N-S lines passing over the borehole site. An E-W trending boundary has been established from these lines with the rocks to the N displaying little or no characteristic reflectors, whilst those to the S exhibit good reflecting horizons. On all of the lines there is some borehole control, six sites having been drilled in the area by m.v. Whitethorn, and from this information a geological succession has been determined. To the S the beds with strong parallel reflectors are Jurassic (Liassic) unconformable upon or faulted against Permo-Triassic. Northward of the boundary the rocks exhibiting no characteristic reflectors are regarded as Palaeozoic, in the case of the borehole 72/60, Tremadocian. The core from this borehole we thus regard as in situ and not, as the authors suggest, a glacial erratic. Prior to Brooks & James I had considered that Carmarthen Bay was floored by Palaeozoic rocks, the Tremadocian outcrop being a submarine extension of the Freshwater East anticline in . Borehole 72/6o lies along the offshore projection of this fold axis. Onshore the oldest rocks in the core of the anticline are Llanvirn in age, no Tremadoc being known from S Pembrokeshire. The Tremadoc therefore represents strata from a deeper level within this structure. The projection offshore of geological boundaries and fold axes in this area is somewhat supported by the sampling (Lloyd et al. I973 Phil. Trans. Roy. Soc. 274A~ 595-626) of what is regarded as ?Carboniferous Limestone S of Caldy Island at the end of refraction line 73/r, along the projected strike of the Carbon- iferous Limestone Series of the Bosherston district. The I.G.S. has a CSP line over these samples and they are regarded as in situ. The authors' work now suggests that a thick Mesozoic succession is present in Carmarthen Bay and that the Palaeozoic rocks in the area S of it represent an E-W trending inlier, flanked to the N and S by post-Carboniferous strata. I feel that Brooks & James's interpretation should be treated with caution until more information in the form of CSP lines and samples is available. The authors themselves state that the use of seismic velocities as indicators are un- reliable on occasion. A velocity determination on the Tremadocian mudstone, admittedly without confining pressure, gave, at the moisture content as received, 3"29 km/sec (axis normal to cleavage) and 4.08 km/sec (axis parallel to cleavage). These are velocities which fit into the Mesozoic sector on their histogram (fig. 8). Also, the CSP line that passes over the Tremadocian borehole terminates a short distance (c. I km) S of refraction line 71/2 without any evidence of Mesozoic rocks. Have the authors attempted similar work and, if so, what were the results ?

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Have they delimited the Mesozoic and attempted sampling in the area? The IGS found a substantial thickness of sand and gravel that was not penetrated by gravity or vibrocores. Have the authors carried out velocity determinations on Triassic rocks? Permo-Triassic mudstones have been cored by IGS (BH 73/38, 39) and underlie the Jurassic. Can these be distinguished from the Jurassic or from the Red Marls of the Old Red Sandstone ? The Helwick Syncline which the authors suggest contains strata of middle or upper Jurassic age has recently been sampled near to its axis by mv Whitethorn (IGS BH 74/28), and dated as lower Liassic. The authors quote a Mesozoic thickness in the vicinity of the fold axis of 700 m. If they could differentiate between the Jurassic and the Permo-Triassic on their refraction lines it would indicate whether a substantial thickness of lower Liassic is present within the structure. In reply to a question from Professor W. W. Bishop, the date of the Tertiary Basin E of Lundy has been determined palynologically by Dr C. Turner as probably middle to upper Oligocene (Fletcher I975, J1. geol. Soc. Lond, this part). Recent work in the Bristol Channel by IGS has indicated the presence of further Tertiary outcrops which are being investigated.

THE AUTHORS thanked Mr Tappin for this additional information and discussion. We do not deny that the core sample from 72/6o could have been derived from in situ Tremadocian lying in the core of an eastward extension of the Freshwater East anticline. Our point is that surface layer velocities along all our refraction lines in the area, when compared with velocities in adjacent parts of the Bristol Channel, suggest an extensive Mesozoic cover and that, if this cover is continuous, it would be wrong to correlate layer 2 along line 73/I with the Tremadocian. The finding of Carboniferous Limestone 3 n miles south of Caldy (Lloyd et al I973) , near to the western end of line 73/I, is certainly compatible with the suggestion that layer 2 may comprise, in part, Carboniferous Limestone and Old Red Sandstone strata. With regard to the suggestion that the Tremadocian may exhibit a velocity similar to Mesozoic strata, ultrasonic velocities derived from a weathered core sample are a very uncertain guide to the velocities derived from large-scale refraction experiments. The University College of Swansea has carried out a limited amount of con- tinuous seismic profiling in Carmarthen Bay but this has not helped in delimiting the Mesozoic outcrop; and the thick sediment layer has precluded gravity core sampling of bedrock. It is therefore an open question as to whether Mesozoic strata occur in the Bay, but the low near-surface velocities along refraction line 7 I/2 certainly suggest that they may do so. No laboratory velocity determinations on Triassic rocks have yet been carried out at Swansea, though recent seismic surveys in and around the inner part of the Bristol Channel suggest that local Triassic velocities fall in the range from 3.oo km/s to 4.5 ° km/s. In general, as stated in the paper, local Triassic and Jurassic strata cannot be differentiated on the basis of velocity. A refraction line in , also referred to in the paper, yielded a velocity of 4"77 km/s for the Red Marls of the Old Red Sandstone.

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The suggestion that the Helwick syncline may include strata of middle or upper Jurassic age was made by M.B. during his talk although not included in the paper. However, the occurrence of lower Lias near to the fold axis is a surprising dis- covery. In view of the similar velocities of Triassic and Jurassic strata, refraction surveys probably could not assist in establishing whether the 7oo m sequence contains a substantial development of lower Lias or is due to a locally thick Permo-Triassic succession. The authors welcome the discovery by the I.G.S. of further Tertiary outcrops in the Bristol Channel and hope that more information will shortly be released.

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