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Home , Carboniferous Limestone, Glamorgan, South Wales, Vale of Glamorgan

Journal ofthe Geological Society, , Vol. 151, 1994, pp. 909-917, 8 figs. 1 table Printed in Northern

Deep structure of the Vale of , South , UK

M. BROOKS, M. MILIORIZOS & B. V. HILLIER Department of Geology, University of Wales, ,PO Box 914, Cardiff CFl 3YE, Wales, UK

Abstract: The deep structureof the is investigated using recently acquired seismic data. Reflectionsections from a commercial seismic survey in theVale enable the , and Silurian sequences to be mapped in the subsurface. The Old Red Sandstone sequence thins westwards under the southern partof the Vale, and in the southwest corner of theVale the base of theOld Red Sandstone is interpretedto overstep across the underlying Silurian sequence in an analogous manner to its regional overstep in . The Precambrian basementsurface is notclearly representedon the sections but maycoincide with thebase of a reflectivesequence observed in placesalong one of the reflectionlines. Basement depth estimates from the reflection sections are compared with estimates derived from a new time term analysis of data from long seismic refraction lines. Basement depths remain uncertain to 1-2 km but appear to be greater under the central part of the Vale than under the western part and along the south coastal zone. A linkedsystem of Variscanforethrusts is tracedto outcrop in thecore of the anticlinebut none involves major displacement. The Ty’n-y-Nant-MoelGilau fault system of the coalfield is shown to bea component of the linked fault system and its current net extension is attributed to Mesozoic reactivation of a Variscan thrust involving only limited displace- ment. The individual thrusts appear to connect to a basal thrust that may extend northwards into or underthe . Theamount of displacementalong the basal thrust cannot be determinedreliably but it mayexceed 10 kmand involvelarge scale repetition of Palaeozoic sequences under the Vale.

The Vale of Glamorgan (Fig. 1) lies immediately south of Minor extensional faults locally preserve Mesozoic strata theSouth Wales coalfield atthe northern margin of the in small outliers.Some of theselate faults are shown as Variscan orogenic belt. Geological map coverage is provided separate structures on the BGS sections but, based on the by BritishGeological Survey (BGS) 1:50000 Sheets 262 demonstration by Miliorizos (1992) of widespead structural () and 263 (Cardiff) published in 1989. Detailed reactivation in the Channel borderlands, we would descriptions of the local geology are presented in expect most to result from reactivation of earlierthrust accompanying memoirs (Wilson et al. 1990; Waters & structures. Lawrence 1987). Previous investigation of the deep geology of the Vale A structural sketch map of the Vale of Glamorgan and involved refraction/wide-angle reflection seismic surveys the adjacent part of the South Wales coalfield is shown in based on land recordings of quarry blasts and marine shots Fig. 2. Figure 3 providesa geological sketch map of the (Brooks et al. 1977; Bayerly & Brooks 1980; Mechie & immediate area of the Shell seismic reflection survey carried Brooks 1984). These surveys identified a high velocity basal out in 1989, the results of which are discussed below. Much refractor, interpreted as a Precambrian crystalline basement, of the Vale is covered by a veneer of Triassic and Jurassic at depths of 4-5 km beneaththe Vale with an overlying strataup to 400m thick thatare largely undeformed but Lower Palaeozoic layer 2-3 km thick beneath the Old Red affected in places by minorfaulting assumed to represent Sandstone. The refraction/wide-angle surveys have recently late reactivation along pre-existing structural lines (Wilson et been extended into southeast Wales (Fig. 1; Hillier 1992). al. 1990; Miliorizos 1992). The pre-Mesozoic geology is Pointers to the concealed Variscan structure of the Vale exposed along the southern margin of the coalfield and in of Glamorgan may be derived from the area severalinliers of Palaeozoic rocks, mainly Carboniferous about 30 km east along the strike of the orogenic belt, and Limestone, displaying large-scale Variscan folding and from the area immediately to the south of faulting (see Figs 2 & 3). the Vale. Williams & Chapman (1986) interpretedthe The main regional Variscan structure is the Cardiff- thrusts and associated fold structures of the Mendip Hills in Cowbridge anticline that trends ENE-WSW across the Vale terms of a thin skinnedforeland thrust model with and exposes Old Red Sandstone and, in the small Rumney northwardtransport and north-southshortening of about inlier north of Cardiff, pre-ORS Silurian strata in its core 20 km (40%). Commercial reflection surveys in the Bristol (Figs 2 & 3). Geologicalsections accompanying the Channel carriedout by GECO-PRAKLA in 1985 provided Bridgend geological map(BGS 1989) show the Cardiff- directevidence for major Variscan thrusting tothe south Cowbridge anticline to be a broad open structure affected by and southwest of the Vale (Brooks et al. 1988; Miliorizos Variscan thrusts of inferred listric geometry and involving 1992). individual displacements of up to a few hundred metres. The The Cardiff-Cowbridge anticline and South Wales thrusts affecting thesouth limb and core region of the coalfield representan anticline/syncline pair with an anticline are shownas an imbricate fan of northward amplitude of about 1 km. To accountfor folding on this directedforethrusts. Thenorthern limb is shown to be scale, Jones (1991) postulated a major foreland-propagated affected by a small number of backthrusts. thrust stack beneaththe Vale,roofed by asouthward- 909

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Fig. 1. Locality map showing multichan- ne1 seismic reflection and wide- , angle/refraction lines in the Vale of Glamorgan.

directedbackthrust at aboutthe level of the base of the nominal 60-fold CMP(common mid point) coverage. ; in this structural model, a thrust Migrated seismic sections were used to investigate the wedge of Old Red Sandstone andLower Palaeozoic strata is subsurface geology. postulated to have driven northwards under the coalfield by The data quality is variable and the seismic sections are at least 10 km, involving an estimated 30-50% shortening. degraded locally by high levels of cultural noise associated, for example, with the and the A48 trunk road Seismic reflection survey in the Vale of Glamorgan that cross the Vale. The complexity of the deep geology may also contribute in places to the poor quality of the sections. A reconnaissance seismic reflection survey was carried out in the Vale of Glamorgan by GECO-PRAKLA for Shell EXPRO in 1989 and the data have kindly been made available by Stratigraphy represented on Vale seismic sections Shell for academicstudy. Two north-south lines 01V and LocalPalaeozoic outcrops and nearbyboreholes provide 02V about 10 km apart extended from near the south coast geological control for the stratigraphic interpretation of the of the Vale northwards into the coalfield and their southern seismic sections. The two nearest deep boreholes arethe portions were linked by an east-west tie line 03V (Figs 1 & and Maestegboreholes in the South Wales 3). The overall length of survey line is about 62 km. The coalfield (Fig. l), drilled in the early 1970sby Cambrian seismic source was four Vibroseisunits with a 25 m SP Exploration Ltd; theseboreholes penetratedto Silurian (source point) interval, anddata were collected using a sedimentary rocks and ?Ordovician volcanic rocks respec- 120-channel spread with a group interval of 25 m giving a tively. The two boreholes indicate Carboniferous Limestone

Fig. 2. Structural sketch map of the Vale of Glamorgan and adjacent areas showing faults and major fold axes, based on British Geological Survey 1 :250 OOO Sheet 51 N-04 W (Bristol Channel). Structures referred to in the text are shown in bold lines and labelled: C-CA, Cardiff-Cowbridge anticline; PS, syncline; RI, Rumney inlier; T/MGF, Ty’n-y-Nant-Moel Gilau fault system; T-y-W, Trwyn-y-Witch fault.

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the line. Seismic marker L (Fig. 4b) can be traced to outcrop in mid-line and represents a horizon at or near the base of the CarboniferousLimestone sequence. The underlying 0.50s interval is characterized by weak, impersistent and irregular reflection events and is interpreted as the Old Red Sandstone interval with seismic marker 0 at its base. The underlying 0.25 S interval is characterized by strong, laterally persistent reflectors and is interpreted as a Silurian sequence containing thin limestone units, similar to the shelf sequences that crop out in the Lower Palaeozoic inliers of the Welsh Borderlands. The base of the Silurian sequence is I assumed to be the regional Upper unconformity (George 1970), represented onthe seismic section by a boundary separating slightly discordant reflector sequences and labelled S on Fig. 4b. Using the velocity information in Table 1, this boundary is calculated to lie at a depth of about 3 km under the southern end of line 02V. Beneath this level, the stratigraphy represented on the seismic sections cannot bedetermined on the basis of available geological controls. TheUpper Llandovery is known to exhibit major overstep within the region (George 1970) and may locally overlie Ordovician (as in the borehole),Cambrian or Precambrian(as in the Malvern Hills and ). The above stratigraphic interpretation of the upper part of the line 02V section can be applied to the other seismic Fig. 3. Geological sketch map of the area of the seismic reflection lines. The Old Red Sandstone interval is observed to thin survey in the Vale of Glamorgan, based on British Geological westwards along line 03V (Fig. 5) from about 1000 m at the Survey 1:50 OOO Sheets 248 () and 262 (Bridgend). intersection with line 02V to about 780 m at the intersection with line 01V. Of more interest, the base of the Old Red Sandstone, which is conformable with the underlying thicknesses of 330m and 510m whereas at Barry, on the Silurian at outcrop in the Rumney inlier of north Cardiff south coast of the Vale of Glamorgan, the Carboniferous (Waters & Lawrence 1987) and under line 02V (Fig. 3b) and Limestone is over1200m thick (Wilson et al. 1990). The the eastern part of line 03V (Fig. 5b), oversteps westwards boreholes indicate Old Red Sandstone thicknesses of about alongline 03V and is represented by a strong angular 970 m and 1310 m north of the Vale and this compares with unconformity under the the southern part of line 01V (Fig. a thickness estimate of about 900m in the Cardiff district 6b). The strong discordant reflectors beneath the unconfor- (Waters & Lawrence 1987). Forthe survey area in the mity in the latter area (labelled R in Fig. 6b) may represent centralVale of Glamorgan,estimated local thicknesses of Silurian sandstones or limestonestruncated by the basal the main stratigraphicintervals down tothe base of the ORS unconformity. Silurian sequenceare given in Table 1 together with the The overstep at the base of the Old Red Sandstone in average seismic velocity for eachinterval as derived from the southwestern part of the Vale is similar to the situation the earlier round of wide-angle seismic surveys in South northwest of the South Wales coalfield where the Old Red Wales (Brooks et al. 1983; Mechie & Brooks 1984). These Sandstone oversteps the entire Lower Palaeozoic sequence thicknesses and velocities are used to predict the two-way from northeast to southwest over a distance of 15 km to lie traveltime (TWT)for reflected waves througheach directly on Precambrian volcanic rocks in the Llangynog stratigraphicinterval, for comparison with reflection times inlier (Cope & Bevins 1993). betweenselected reflectors representedon the seismic Overstep by theOld Red Sandstone in the Vale of sections. Glamorgan has relevance to the question of the source area The stratigraphy of the upper part of the three seismic for the Lower Old Red Sandstone Llanishen Conglomerate sections is best displayed on line 02V (Fig. 4). The of the Cardiff district. This southerly derived conglomerate uppermost seismic layer (notmapped on the section) unit (Allen 1975; Waters & Lawrence 1987) contains clasts extends to a two-way reflection time of about 0.05 S and of Llandoveryquartzite and acid volcanic rocks and the represents flat-lying Mesozoic strata up to 100 m thick along seismic sections suggest thatthere were local outcrops of Silurian rocks in the Vale area in early Devonian times. Table 1. Estimated thicknesses andvelocities formajor stratigraphic units in the central Vale of Glamorgan area Structural interpretation of Vale seismic lines Unit Thickness Velocity Predicted TWT North-south lines 01V and 02V providedip sections (m) (km S-') (S) through the prevailing Variscan structure but exhibit in places complexity and poor signal-to-noise ratio that Carboniferous Lst 800 5.2 0.31 Old Red Sandstone 0.43loo0 4.7 preclude a complete structural interpretation. Silurian 400 5.5 0.15 The structure is best imaged on line 02V (Fig. 4). In its upper part, the seismic section along line 02V reveals the

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03V Penmark Cowbridge S Wales I syncline anticline coalfield

0-

1-

2-

3-

(a1 a

0Y I I ...... '. . , ', '.'. : . ,:.: .. .:: : . .. . , ....,.

0 5 10 km I 1 I Fig. 4. (a) Seismic section along Vale reflection line 02V. (b) Line interpretation of section with a strip showing a simplified version of the surface geology (geological symbols as shown in key to Fig. 3; C-CA, Cardiff-Cowbridge anticline; T/MGF, Ty'n-y-Nant-Moel Gilau fault). L, 0 and S are interpreted to be the baseof the Lower Carboniferous, ORS and Silurianrespectively.

Cardiff-Cowbridge anticline to bea simple open fold there is evidence for structural discordances displacing the structure as imaged by the basal Carboniferous Limestone basal ORS unconformity and underlying Lower Palaeozoic seismic marker L (Fig. 4b). The structureis succeeded to the units. south by acomplementary syncline in Palaeozoic strata Aproblem with the interpretation of the deeper concealed beneath the local Mesozoic cover. This syncline discordances in terms of Variscan faulting is the possible (Fig. 2) is representedon the BGS structuralsections existence of earlierstructures as observed in Lower accompanying the BridgendSheet and is named the Palaeozoic outcrops in nearby areas such as Pembrokeshire Penmark syncline by Miliorizos (1992). Atgreater depth (Powell 1987, 1989) and the Malvern Hills (Brooks 1970).

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W VALE OF GLAMORGAN LINE 03V E

01v 02v I I 0-

1-

2-

C b) 0 5 10 km

Fig. 5. (a) Seismic section along Vale reflection line 03V. (b) Line interpretation of section.

Thestructural interpretation lines shown in Fig. 4b are show little orno netdisplacement of the basal ORS, restricted to faults of Variscan and younger age. Whilst the indicating that any Variscan shortening is balanced by later possibility of deeper Variscan structurescannot beruled reactivation in extension. out, those shown in Fig. 4b define athin-skinned tectonic Fault D shows net extension in the Carboniferous section system of linked Variscan forethrusts. Faults A, B and C cut at the northern end of the seismic line and reaches surface in up to surface in the Old Red Sandstonecore of the the vicinity of the Ty’n-y-Nant-Moel Gilaufault system Cardiff-Cowbridge anticline in an area of partial Mesozoic (Fig. 2), a zone of mainly southward-dipping normal faults cover. Fault A appears to displace the basal ORS by about and associated north dippingantithetic structures. The 350 m and may be correlated with an outcropping thrust of Ty’n-y-Nant-Moel Gilau fault system post- hasa similar displacement shown onthe Bridgend 1:50 000 Carboniferous vertical throw exceeding 1100 m about 15 km geological mapand the accompanying structural section to the west, but amounting to only a few hundred metres in along a nearby north-south line (see Fig. 4). Faults B and C the vicinity of the seismic lines (BGS 1:50 000 Sheet 248

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03V Cowbridge 03V S Wales I anticline coalfield

tal LL .zU l- A I ..... b ...... k...... I......

0-

1-

2-

3- 0 5 10 km

Fig. 6. (a) Seismic section along Vale reflection line 01V. (b) Line interpretation of section with a strip showing a simplified version of the surface geology (geological symbols as shown in key to Fig. 3: T-y-WF. Trwyn-y-Witch fault; C-CA. Cardiff-Cowbridge anticline; T/MGF, Ty'n-y-Nant-Moel Gilau fault). R is explained on p. 91 1.

(Pontypridd), 1963; Woodland & Evans 1964). The absence ment, and it must either die out rapidly northwards or, more of significant shortening of theUpper Palaeozoic section likely, extend into or under the coalfield in the footwall of across faults A-D means that Variscan thrust displacements the Ty'n-y-Nant-Mod Gilau fault system. on these structures must have been small in scale. only of On line 03V (Fig. S). near its intersection with line 02V, the same order of magnitude as post-Variscan extension. a strong planar reflection event at 1.7 S TWT correlates well Faults B, C and D appear to flatten into a basal thrust with the basal thrustmapped on line 02V. However,this beneaththe base of the Silurian thatcan be traced reflection cannotbe traced further west along line 03V southwards to 1.5 S TWT before it is lost in a zone of low because it dies out laterally into a zone of low signal-to-noise ratios.This thrust is of unknown displace- signal-to-noise ratio.

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In the core of the Cardiff-Cowbridge anticline, line 01V shots and recording sites established by Mechie & Brooks (Fig. 6) displays a confused section of laterally discon- (1984) for time term analysis of basement depths. tinuous, arcuate reflection events through which no discrete The new estimates of basement depth under the Vale fault structures can be traced. The filtered stack section of derivedfrom the extendedtime term interpretation are the same zone is dominated by diffraction hyperbolae and shown in Fig. 7. Along the south coast of the Vale, near to similarly reveals no discretestructures. The Ty'n-y-Nant- theend of reflection lines 01V and 02V, basement is Moel Gilau fault system (Fault D, Fig. 6b) is poorly imaged estimated to lie at depths of 4.7-5.8 km in the hanging-wall under thenorthern part of the line,but its southward of the basal thrust. Thesedepths are greaterthan the continuation may be marked by laterallydiscontinuous estimates of 3.7-4.2 km based onthe earlier time term reflection events traceable to 1.4s TWT in the core of the analysis butoverlap with depth estimatesderived from Cardiff-Cowbridge anticline.Fault D involves an exten- interpretation of wide-angle reflection data from the nearby sional displacement of 100-200m at the base of the Cornelly- quarry blast line (Figs 1, 7; Bayerly & Carboniferous Limestone. Brooks 1980). Giventhat the basal Silurian occurs ata The basal Carboniferous seismic marker L can be traced depth of about 3 km under the southern endof line 02V, the almost continuously across the line 01V section (Fig. 6b). Its pre-Silurian sequence overlying basement in the hangingwall simple geometry precludes any large faulted displacements block is estimated to be about 1.7-2.8 km thick. and shows that no major Variscan thrusts cut up to surface Using the sameaverage supra-basement velocity of in the core of the Cardiff-Cowbridge anticline. Two minor 5.3 kms -' employedin the timeterm interpretation, any backthrusts occupy the south limb of the anticline along the near-normal incidence reflection event from basementat line of the Trwyn-y-Witch fault (Figs 2, 3). 4.7-5.8km under the southern end of the reflection lines would be expected to occur at 1.8-2.2 S TWT. In fact, there is no discrete eventon the reflection sections that can Basement depths under the Vale of Glamorgan readily be interpreted as a basement reflection. Previouswide-angle/refraction seismic lines in the region Strong reflection events are observed further north along (Fig. 1; Mechie & Brooks 1984) provided depth estimates to line 02V to 2.5 S TWT with weaker events to about 2.8s a 6.0 kms-' basal refractor interpreted asPrecambrian TWT (Fig. 3). Underthe centralpart of the line, strong crystalline basement.Horizontal layer interpretation of north-dipping reflection events can be traced to 2.8s TWT. If seismic line D (Fig. 1; Brooks et al. 1977) predicteda the base of these reflective sequencesrepresents the basement depth of about 5 km along the south coast of the minimum possible depth to basement, the reflection time of Vale. Raytrace interpretation of wide-angle reflection 2.8 S indicates a local basement depth of at least 7.4 km, events identified onthe Cornelly-Aberthawquarry blast about2 km deeper than the nearesttime term depth line (Fig. 1; Bayerly & Brooks 1980) producedbasement estimate along the south coast of the Vale. depth estimates in the range 4-5 km beneath the southwest Major tectonic repetition under the Vale could explain part of the Vale. such achange in predictedbasement depth and would Mechie & Brooks (1984) compiled time-distance data require the basal Silurian to recur in the footwall block. A from a regional network of quarry blasts, marine shots and basal Silurian eventcannot be identified reliably in the land recordingsites to carryout a time term analysis of footwall block along line 02V but, on the basis of broadly basement depths across the SouthWales/Bristol Channel similar reflection sequences in the hanging-wall and footwall region. Time term analysis produces individual estimates of blocks under the Penmark syncline (see Fig. 3b), it might refractor depthunder eachshot and recordingsite in the occur at about 2.0s TWT, equivalent to a depth of about network.When applied to deep refractors, the method 5.3 km. Deducting this value from a local basement depth of involves an inherent smoothing of basement relief features at least 7.4 km produces an estimated thickness of 2.1 km or over a lateral distance of several kilometres from each site more for the pre-Silurian sequence, in 'generalagreement but nevertheless enables large scale basement features to be with theinterpeted thickness 1.7-2.8of km in the defined. Mechie & Brooks (1984) estimatedbasement depths of 3.7-4.2 km along the southern coast of the Vale and 4.6 km at Cornelly Quarry in the western part of the Vale (Fig. 1). During the period 1989-1991, further long seismic lines were established in the Vale of Glamorgan and the eastern half of the South Wales coalfield (Hillier 1992). These lines were subjected to ray trace interpretationand selected time-distance data were added to the regional time term dataset established by Mechie & Brooks (1984). The overall results of this deep seismic study will be reported elsewhere (Hillier, in prep).Here, revised estimates of basement depths under the southern part of the Vale of Glamorgan are compared with uncertainestimates of basement depth derived from the reflection sections. The new refractionlwide-angle survey included two profile lines recorded across the Vale of Glamorgan and the SouthWales coalfield froma common shot point off the Vale coast (Hillier 1992). Selected time-distance data from Fig. 7. Basement depths in the southwest part of the Vale derived these lines have been used to extend the regional network of from time term analysis.

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hanging-wall block (seeabove). Conversely, if there is no (Careg Cennen-Church Stretton, , Valeof significant tectonicrepetition underthe Vale, thedeeper ; Owen & Weaver 1983) traversing the South Wales basement under the central part of line 02V would require coalfield. Whilst thelatter are conventionally regardedas the pre-Siluriansequence overlying basement to thicken overlying basementstructures of great longevity, they all rapidly northwards tomore than 4 km underthe central demonstrate compressional tectonics of Variscan age though Vale. the amount of Variscan displacement across them is in no The revised time term analysis indicates basement depths way comparable with the possible displacement of 10 km or of 3.6-4.5 km under the western part of the Vale (Fig. 7) more along the basalthrust under the Vale. There is, with an individual depth estimate of 3.9 km a few kilometres however, the possibility of agradual loss of displacement west of reflection line 01V. Basement therefore appears to northwards along a combination of the basal thrust and any shallow westwards from line 02V, but the overall linked structures. configuration of the basementsurface underthe Vale A second problem is the whereabouts of any major basal remainsunclear; the line 01V and 03V sections show no thrust along the regional strike to east and west. Westwards discrete deep reflection patternsthat can be reliably any basal thrust must intersect, along the west coast of the interpreted in terms of local basement depths. Vale of Glamorgan, with the northwestward extension of the Cothelstone fault, interpeted by Miliorizos (1992) as a transferfault zone compartmentalizing Variscan thrust Discussion of the subsurface structure of the Vale sheets. Eastwardsthe thrust may connect either with the Structural interpretation of the reflection lines demonstrates basalthrust of the Mendip Hills, the concealed structure that Variscan thrusts can be mapped in the subsurface but postulated by Williams & Chapman (1986) to underliea that thosereaching surface have only small net displace- wide area around the inner including the Usk ments andcannot haveundergone large thrust displace- and Tortworth inliers and the syncline, or ments during Variscan deformation. The listricity of the with a deeper detachment surface. individual thrusts and their small displacements accord with Finally, it needs to be established whetherthe theBGS structural sections accompanying the Bridgend hanging-wall block containsPrecambrian at shallow depth Sheet. Differences from the BGS sections include the linked underthe south coast of the Vale of Glamorgan. The nature of the thrust system andthe existence of a basal possible northward fall in basement level from about 5 km thrust that probably extends into or under the South Wales along the south coast of the Vale to over 7 km under the coalfield and may involve majordisplacement. The central part of line 02V may result from major displacement interpreted thrustsystem forthe Vale shows geometrical of thebasement surface along the basal thrustas shown similarities with the thin skinnedthrust model forthe schematically in Fig. 8. Mendip Hills some 30 km to the east along the strike of the It is important to note that if the basement surface is Variscan fold and thrust belt (Williams & Chapman 1986). affected by major thrusting as shown in Fig. 8 the resultant In contrast with the structural model of the South Wales complex geometry of the basement surface will locally coalfield presented by Jones (1991, fig. 10), the Variscan invalidate the assumptions underlying the time term analysis thrusts areinterpreted as an imbricate fan ratherthan a of basement depth. This point, which is discussed in more triangle zone and individual thrustdisplacements are of detail in Hillier (1992), highlights the problem of attempting much smallermagnitude. An essential difference is that to derive accurate basement depths from time term analysis. Jones’s model requires a zone of major backthrusting along Shallower estimates of basement depth under the southern the southern margin of the coalfield at about the base of the Carboniferous Limestone. No such major backthrusting has been recognized in recent geological mapping by the BGS S N (Waters & Lawrence 1987; Wilson et al. 1990) andthe seismic sections provide no subsurface evidence for it. If any trianglezone roofed by a passive backthrust of largedisplacement exists underthe Vale of Glamorgan it must occur at a lower stratigraphic level than postulated by Jones (1991) and, in any case, no major duplex structures of the type illustrated by Jones (1991, fig. 10) are imaged in the seismic sections.Across the Cardiff-Cowbridge anticline, shortening in the Silurian to Carboniferousinterval is negligible, whereas Jones (1991) postulatedshortening of 30-50% due to thrust stacking within a stratigraphic interval including the Old Red Sandstone and part of the underlying Lower Palaeozoic. The possibility of a major basal thrust under the Vale of Glamorgan raises a number of problems about the regional Variscan structurethat provide animportant focus for undmerematedS~PI~-LW~B~I future structural and geophysical investigations. 0 5krn The first such problem is how any basal thrust involving major displacement would link with thrust structures in the Fig. 8. Sketch section across the basal thrust under the Vale of South Wales coalfield whose individual displacements are Glamorgan based on interpretation of seismic section 02V (Fig. 4), typically smaller by more than a factor of ten. It is possible simplified to show the possible effect of thrusting on basement that the basal thrust links with the caledonoid fault zones elevation.

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part of the Vale, similar to those obtained by Mechie & BROOKS,M. 1970. Pre-Llandoverytectonism and the Malvern structure. Brooks (1984), are derived when datafrom raypaths Proceedings of the Geologists’ Association, 81, 249-268. -, BAYEKLY,M. & LLEWELLYN,D. J. 1977. A new geological model to traversing the zone of basement thrusting are excluded from explain the gravity gradient across Exmoor, north . Journal of the the analysis, hence accentuating the predicted differences of Geological Society, London, 133,385-393. basement level. -, MECHIE,J. & LLEWELLYN, D.1983. J. Geophysical investigations in the In the absence of any direct evidence from the surface Variscides of southwest Britain. In: HANCOCK,P. L. (ed.) The Variscan fold belt in the British Isles. Hilger, Bristol, 186-197. geology or seismic sectionsfor Jones’s (1991) major -, TKAYNEK,P. M. & TKIMBLE,T. J. 1988. Mesozoic reactivation of backthrust and associated triangle zone under the south crop Variscan thrusting in the Bristol Channelarea, UK. Journal of the of the coalfield, it is importantto explorealternative Geological Society, London, 145, 439-444. structuralexplanations for the 2 km elevation of the COPE,J. C. W. & BEVINS,R. E. 1993. Thestratigraphy and setting of the Precambrian rocks of the Llangynog inlier,, South Wales. CarboniferousLimestone of the Vale aboveits level Geological Magazine, 130, 101-111. beneath the South Wales coalfield. If the Vale is in the GEORCE,T. N. 1970. British Regional Geology: South Wales (3rd edn). hanging-wall block of a major basal thrust, the geometry of HMSO, London. the thrust surface is likely to be the primary cause of the HILLIER,B. V. 1992. Seismic studies of deep structure beneath the South Wales location and amplitude the Cardiff-Cowbridge antidine. coalfield and adjacent areas. PhD thesis, University of Wales. of JONES,J. A. 1991. A front model for the Variscan deformation of Existing seismic data are inadequate to determine the actual theSouth Wales coalfield. Journal of the GeologicalSociety, London, geometry of the thrust. 148,881 -89 1. From the above itis clear that majoruncertainties MECHIE, J. & BROOKS,M. 1984. A seismic study of deep geological structure remain regarding thedeep structure of the Vale, in in the Bristol Channelarea, SW Britain. Geophysical Journal of the Royal Astronomical Society, London, 78, 661-689. particular, the scale of the basal thrusting and the detailed MILIOKIZOS,M. 1992. Tectonic evolution of the Bristol Channel borderlands. configuration of the basementsurface. These problems PhD thesis, University of Wales. cannot be resolved without further and more detailed OWEN,T. R. & WEAVER,J. D. 1983. The structure of the main South Wales seismic investigations in the Valeand the immediately Coalfield and its margins. In: HANCOCK,P. L. (ed.) The Variscan fold belt in the British Isles. Hilger, Bristol, 74-87. surrounding areas. POWELL,C. M. 1987. Inversion tectonics in SW Dyfed. Proceedings of the Geologists’ Association, 98, 193-203. - 1989. Structural controls on Palaeozoic basin evolution and inversion in We thankShell EXPRO Ltd for kindlymaking the Vale seismic southwest Wales. Journal of the GeologicalSociety, London, 146, sectionsavailable to thisstudy, and for theaward of aresearch 439-446. studentship to M.M. The Natural Environment Research Council is WATERS,R. A. & LAWRENCE,D. J. D. 1987. Geology of the South Wales thanked for theaward of aresearch studentship to B.V.H. The Coalfield, Part 111, the country around Cardiff. British Geological Survey constructivecomments of thetwo reviewers enabledthe original Memoir for 1 :SO OOO geological sheet 263 ( and Wales). HMSO, manuscript to be much improved. London. WILLIAMS,G. D. & CHAPMAN, T.1986. J. The Bristol-Mendip foreland thrust belt. Journal of the Geological Society, London, 143, 63-13. WILSON,D., DAVIES,J. R., FLETCHER,C. J. N. & SMITH,M. 1990. Geology of References the South Wales Coalfield, Part VI, the country around Bridgend. British Geological Survey Memoirfor 1 :SO000 geological sheet 261 and 262 ALLEX,J. R. L. 1975. Sourcerocks of theLower Old Red Sandstone: (England and Wales). HMSO, London. Llanishen Conglomerate of the Cardiff area, South Wales. Proceedings of WOODLAND,A. W. & EVANS,W. B. 1964. Geology of the South Wales the Geologists’ Association, 86, 63-76. Coalfield, Part IV, the country around Pontypridd and Maesteg. British BAYEKLY,M. & BROOKS.M. 1980. A seismic study of deep structure in South Geological Survey Memoir for 1 :63360 geological sheet 248 (England Wales. Geophysical Journal of the Royal Astronomical Society, 60, 1-20 and Wales). HMSO, London.

Received 24 June 1993; revised typescript accepted 31 March 1994

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