Deep Structure of the Vale of Glamorgan, South Wales, UK
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Journal ofthe Geological Society, London, Vol. 151, 1994, pp. 909-917, 8 figs. 1 table Printed in Northern Ireland Deep structure of the Vale of Glamorgan, South Wales, UK M. BROOKS, M. MILIORIZOS & B. V. HILLIER Department of Geology, University of Wales, Cardiff,PO Box 914, Cardiff CFl 3YE, Wales, UK Abstract: The deep structureof the Vale of Glamorgan is investigated using recently acquired seismic data. Reflectionsections from a commercial seismic survey in theVale enable the Carboniferous Limestone, Old Red Sandstone 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 west Wales. The Precambrian basementsurface is notclearly representedon the sections but maycoincide with thebase of a reflective sequence observed in places along one of the reflection lines. 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 Cowbridge anticline but none involves major displacement. The Ty’n-y-Nant-MoelGilau fault system of the South Wales 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 South Wales coalfield. 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 (Bridgend) and 263 (Cardiff) published in 1989. Detailed reactivation in the Bristol 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 beneath the Vale with an overlying strata up to 400m thick that are 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 Mendip Hills 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 Bristol Channel 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 to the 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/6/909/4890096/gsjgs.151.6.0909.pdf by guest on 02 October 2021 910 M. BROOKS. M.MILIORIZOS & B.V. HILLIER 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. Carboniferous Limestone; 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 M4 motorway 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 academic study. 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 & Senghenydd 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, Penmark syncline; RI, Rumney inlier; T/MGF, Ty’n-y-Nant-Moel Gilau fault system; T-y-W, Trwyn-y-Witch fault. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/151/6/909/4890096/gsjgs.151.6.0909.pdf by guest on 02 October 2021 DE EP STRUCTUREDEEP OFVALE THE OF GLAMORGAN 91 1 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 Llandovery 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 Maesteg borehole),Cambrian or Precambrian(as in the Malvern Hills and Pembrokeshire). 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.