Journal of the Geological Society, London, Vol. 145, 1988, pp. 915-923, 3 figs Printed in Northern Ireland

Is the Welsh Borderland Fault System a terrane boundary?

N. H. WOODCOCK’ & W. GIBBONS’ Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK Department of Geology, University College, Cardiff,Cardiff CF1 lXL, UK

Abstrna The Welsh Borderland Fault System forms one of the most prominent tectonic lines in Southern Britain and, along with the Menai Straits and Malverns Fault Systems, is a candidate for a terrane boundry. A review of straiigraphic linkage across the system reveals significant contrasts in pre-Ashgill sequences. These contrasts occur particularly across its north-west edge, the Pontesford and Tywi Lineaments, rather than across the Church Stretton Lineament. The Ashgill event produced strike-slip dominated structure and an unconformity that expands from the Welsh Basin towards the platform. An important linkage sequence was produced by the subsequent late Ashgill to Llandovery marine transgression across the fault system. Silurian facies changes are marked but gradational, and no more than expected across a basin margin from stable to thinned crust. Devonian(Acadian) sinistral transpression, and later Phanerozoic movements, reactivated the faultsystem but didnot engineer major terrane amalgamationacross it. Thisreactivation has obscured the early history of thefault system. The remaining evidence admits, though does not prove, majorAshgill or earliertranscurrent movements. Broader considerations suggest that the Welsh BorderlandFault System originated either as a Cambro-Ordovicianextensional system on the south-eastern margin of the Welsh Basin or during Late Precambrian-Early Cambrian transcurtent movements occurring within Avalonia, inboard from the Menai Strait line.

Recent application of the displaced terrane concept to stratigraphic gradients across the fault system. The main two Caledonian geology has focused attention on themajor transects, at the latitude of the Shelve/LongmyndInliner steep fault systems thatseparate areas in Britain with (aa’, Fig. 2) andthe BuilthInlier (bb’, Fig. 2) show differing pre-Devonian geology (Fig. la: e.g. Gibbons & relationships only mildly affected by Variscan deformation. Gayer 1985; Hutton 1987). Some of these faults such as the For comparison a third transect (cc’, Fig. 2) shows the effect Highland Boundary Fault (Bluck 1984) and the Menai Strait of Variscan overprint on a N-S profile through SW Wales Fault System (Gibbons 1987) are fundamental. basement (Pembrokeshirecoast). All transectdiagrams are drawn lineaments, early movements along which have juxtaposed with the same vertical time scale, to emphasise the major such different geological units that a compelling case can be unconformities that might bracketany terrane amalgama- made for interpretingthem as majorterrane boundaries. tion event. Other fault systems may have originated as terrane boundaries, but the evidence for both age and direction of earliest movements has been obscured by reactivation. Pre-Ordovician linkage A terrane boundary may be defined as a fault or shear The oldest rocks exposed within the WBFS occur as zone,movements along which have juxtaposedtwo basement inliets along and between the Pontesford-Linley geological units that were previously unrelated. Such and Church Stretton Fault Zones (transect aa’, Fig. 2; Greig boundaries are usually the focus for later reactivation, and it et al. 1968). These inliers expose volcanic rocks (Uriconian is therefore the early fault displacements that are of critical Group) and younger sediments (Longmyndian Supergroup). importance.Where a major fault system is suspected of Uriconian volcanics have yielded an Rb-Sr WR age of being a terrane boundary at some time in its displacement 558 * 16Ma (Patchett et al. 1980), whereas a basaltic dyke history, this suspicion can only be removed by proving some intrusive intothe Uriconianhas produced K-Ar ages of form of linkage across the fault at that time. Such linkages 632 f32Ma and 677 f 32 Ma.Radiometric ages on the may be purely lithostratigraphic, or may depend on other Precambrian in southern Britain are, however, to be viewed lines of evidence, such as igneous or palaeomagnetic events. with some uncertainty given the present problems emerging This paper examines thesteep fault system that runs from the anomalously young Rb-Sr ages obtained for the NE-SW through the Welsh Borderlands.This Welsh supposed crystallization of the Ercall Granophyre (Cope & BorderlandFault System (WBFS) is defined by the Gibbons 1987). Of more definitevalue is a minimum Pontesford, Tywi and Church Stretton Lineaments. These stratigraphic age forthe basementprovided by Lower anastomose with other lines of brittle and ductile Cambrian sediments (Wrekin Quartzite Formation) that rest deformation to form a network of structures running down unconformably upon the Uriconian (Greig et al. 1968). intoSouth Wales (Fig. lb). Thisfault system has The Uriconian and Longmyndian rocks form part of the undoubtedlyhad a longmovement history, but it is less Late Precambrian to early Cambrian basement to southern clear whether the system was ever a terrane boundary and, Britain.This basement is dominated by plutonic, volcanic if so, when the major displacements took place. andsedimentary rocks that have yielded radiometricages Three referencetransects are used to illustrate ranging from c. 700-530 Ma (see review by Thorpe et al. 915

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Great Glen F.

/?A--- / 4 10

Fig. 1. (a) Major faults systems in Britain, that separate areas of distinctive Precambrian or Lower Palaeozoic geology. Some of these systems are interpreted as terrane boundaries (see text). Codes:1 = Northwest Foreland ( = Laurentian Craton), 2 = Moine Terrane, 3 = Grampian Terrane (Dalradian and Central Highland Division), 4 = Midland Valley Terrane, 5 = Southern Uplands Terrane, 6 = Lake District/Isle of Man, 7 = Monian Terranes, 8 = Welsh Basin, 9 = Midhd Platform, 10 = North Armorican area (after Gibbons & Gayer 1985). (b) Major fault belts in Wales mentioned in the text.

1984). Despiteconsiderable lithological and geochemical (1986) makes a case for late Precambrian/early Cambrian variation between different inliers, thereare many broad faulting in the Longmynd area.The Malvernianplutonic similarities. Parts of the clastic Longmyndian succession for rocks are apparently over100Ma older than other example, have astrong volcanic influence similar tothat borderland inliers (Beckinsale et al. 1981), and it is seen in the CharnianSupergroup of Central England therefore simplistic to refer all these rocks to a single (Moseley & Ford 1985; Pauley 1986). An Ediacaran fauna ‘Cadomian’ orogenic event. has been described both from the Charnian and similar late The correlation of Precambrian rocks across the Church Precambrian rocks in South Wales (Cope 1977). Most of the Stretton Fault (Fig. 2a) provides a linkage which implies igneous rocks exhibit a geochemistry compatible with an that any major displacements on the WBFS occurred on the island arc origin (Thorpe 1982) and a gross similarity with Pontesford or Tywi Lineaments further northwest. Longmy- coeval rocks in the Avalon Terrane of Newfoundland has ndian sediments can certainly be matched between the main led to the correlation of the two areas (e.g. Kelnnedy 1979; outcrop, NW of the Church Stretton Fault Zone, and fault Gibbons 19836). However it shouldbe emphasized that bounded slivers within the zone itself (Greig et al. 1968; there is a great age difference between some of these areas, Pauley 1986). The sediments in these slivers sit unconform- e.g. the Malvern plutonic rocks. ably on Uriconian rocks that can be matched broadly with The three most prominentfault systems known to cut Uriconian outcrops tothe southeast of the faultbelt. Precambrian basement in southern Britain are those of the Aeromagneticmaps suggest more extensive buried Urico- Menai Strait, the Welsh Borderland and the Malverns (Fig. nian here (Wilson 1980). Detailed correlation of the highly 1). The Menai Strait Fault System has been interpreted as a variable Uriconiansequences between separateareas is transcurrent fault active immediately prior to the deposition difficult (Greig et al. 1968), but differences are not of late Lower Cambrian sediments in North Wales (Gibbons considered significant enough to justify placing major 1987). The Malvernian plutonicbasement rocks are also tectonic boundaries between them. All the Uriconian rocks reworked by steeply dipping, NE-SW striking ductile fault are therefore placed in one terrane. rocks; e.g. the wide zone of granite mylonites exposed just The nature of the Precambrianbasement between the south of the Wyche Cutting in the Malverns (Thorpe 1988). Pontesford-Linley and Bala Faults is unknown. The oldest These mylonites are presumably olderthan thle Cambro- rocks between the Bala and MenaiStrait line are (1) the Silurian cover sequence, and may relateto the Monian volcanic rocks in the Bryn-Teg borehole, Harlech Dome terrane docking events further northwest. NO undoubtedly (Allen & Jackson 1978), (2) the Arfon Group volcanic rocks early fault rocks are known from the WBFS, though Pauley beneath the Lower Cambriansediments of NW Wales

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(Reedman et al. 1984) and 3) the poorly exposed plutonic thick Llanvirn to ?Caradoc volcano-sedimentary sequence in rocks and gneisses of the Sarn Complex, immediately SE of the Builth Inlier lies within the fault belt of the Pontesford the Llyn shear zone (Beckinsale et al. 1984; Gibbons 19836, Lineament (Woodcock 19846, 19876). The shallow marine 1987). Any or all of these rock types could belong to the to subaerialenvironments here(Jones & Pugh 1949) Avalonian basement. contrast with deeper marine Caradoc to Ashgill sediments Although it seems likely that the Avalonian basement of across the Tywi Lineament to thenorth-west (Roberts 1929). southern Britain was dissected by early faults prior to the Large post-Caradoc displacements could be accommodated Early Cambrianmarine transgression, there is as yet no on faults on either margin cf. the Pontesford Lineament, control on the direction or amounts of any displacements. though itsnorthwestern margin if now more strongly The broad lithological similarities between all basement expressed, partly through post-Silurian reactivation (Wood- rocks SE of the Menai Strait line allow them all to belong to cock 19846). a single Avalonian terrane. Whether this terrane is in fact composite remains to beproved. If largeearly displace- ments occurred on the WBFS, they were probably taken up Ashgill tectonics on the Pontesford and, or, Tywi Lineaments rather than on The timing of any major Ordovicianfault displacements the Church Stretton line. along the WBFS is constrained within the ‘Ashgill’ unconformity of Fig. 2. The base of the overstep sequence varies in age from mid Ashgill to Wenlock,whereas the Ordovician linkage sub-unconformity sequences range high in the Caradoc. A There is stronga contrast in preservedOrdovician more detailed view of Ashgill stratigraphy (e.g. Williams et sequences across the WBFS between the Midland Platform al. 1972) reveals two unconformities. The lower one occurs to the SE and the Welsh Basin to the NW. The platform has in earliest Ashgill (Pusgillian to early Cautleyan) time and a thin veneer of Caradoc shallow marinesediments probably marks the tectonic event along the WBFS and unconformable on Tremadoc or earlier rocks. Th.e basin has more widely across Wales. The upper one occurs in latest thickerand more complete sedimentary sequences, com- Ashgill (late Hirnantian) time and coincides with the monly intercalated with volcanics of Tremadocto Early well-established glacioeustatic fall in sea level (Brenchley & Caradoc age (reviewed by Allen 1982). Extensive Newall 1984, Woodcock & Smallwood 1987). unconformities occur in the basin only below the Arenig and Course clastics weresupplied laterally across the Tywi below the Ashgill. There is uncertainty aboutthe Lineament in mid Ashgill (Camlo Hill group, Roberts 1929) pre-Ashgill sequence over a large area of Mid ’Wales. The probably sourced from uplift along the WBFS. Similar facies best guess is that this area is floored by volcano-sedimentary are seen further south-west in the Llandovery district rocks similar to those plunging under it fromN (Smallwood 1986). Further coarse clastics nearthe Pembrokeshire at one end or from the Benvyn Hills at the Ashgill/Llandovery boundary(e.g. Cerig Gwynion Grits; other.The Ordovician volcanic areas close tothe basin Kelling & Woollands (1969), and underlying Ashgill margin at Llanwrtyd, Builth, Shelve and the Breidden Hills arenites) may havebeen triggered instead by the are not a reliable guide to the basin centre geology, given glacioeustatic sea level changes(Woodcock & Smallwood the possibility of significant later displacements onthe 1987). intervening Twyi Lineament or intra-basin lineaments. The main structuralevidence for an Ashgill tectonic Transect aa’ (Fig. 2) details thestrong shelf/basin event comes from along the Pontesford Lineament, and the contrast. Animportant fault-bounded sliver along the other two elements of the WBFS may have been less active Pontesford Lineament with Caradoc unconfmmable on at this time. A strong strike-slip component is suggested by Precambrian is regarded as the north-westernmost remnant the arrays of steep, dominantly strike-slip faults in both the of the Midland Platform stratigraphy (Whittard 1952). The Builth and Shelve Inliers. These have been tentatively Caradoc rocks contain faunasand lithologies Identical to interpreted as part of a major dextral system by Woodcock those of the Church Strettonarea to the SE: (Dean & (19846) and in moredetail by Whittard (1979) andLynas Dineley 1961). On this transect the Pontesford-Linley Fault (1988) at Shelve, and Woodcock (1987~)at Builth. Pauley marks an abrupt change tothe much more complete (1986) argues fora sinistral Precambrian origin for steep sequence of the Shelve area. Thisfault is therefore often fault arrays onthe Long Mynd,bordering the Church taken as the south-eastern boundary of the Welsh Basin in Stretton Fault, but these faults could also have Ashgill and Ordovician time (e.g. George 1963; Dean 1964; ‘Wills 1978). later components. However the shelf/basincontrast has beenenhanced by Offsets of fold axial traces and lithological outcrops in tectonic omission of transitional facies during late the Shelve area suggest dextral offsets on some individual Ordovician strike-slip displacement (Woodcock 1.9846). The fault strands of theorder of akilometre (Lynas, 1988), magnitude of this displacement is unconstrained here. There though stratigraphic and structural mismatches across the is a less marked contrast in Ordovician geology on transect Whitegrit Fault and the Pontesford-Linley Fault imply aa‘ (Fig. 2) across the Severn Valley Faults, a geometrical displacements on eachstrand of morethan 10 km. Large continuation of the Tywi Lineament. Caradoc: sequences total displacements during the Ashgill event are therefore across this line could have beenpart of thesame admissible along the Pontesford Lineament. However if the volcanically-influenced marine shelf setting. Builth and Shelve areas, now partly on opposite sides of this Transect bb‘ (Fig. 2) shows analogous c:ontrasts in lineament, were elements of the same volcano-sedimentary Ordovician geology to transectaa’. Here however the centre then finite displacement may only have been of the platform sequence lacks Ordovicianrocks, with Silurian order of their present separation, about 40 km (Woodcock sediments lying unconformably on Precambrianand 1984~). TheAshgill faulting event was accompanied only by Cambrian inliers along the Church Stretton Lineament. The open folds and sporadic weak cleavage, suggesting that it

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C 30(

l\. . . ..I.. . . .

35c

400

MARINE FACIES

shallow NON-MARINE FACIES

4 50 B carbonates m continental

500 deep sands Precambrian S] m muds not to scale 0 Ma v) unco n - I I formity lOkm

Fig. 2. Space vs, time diagrams for three transects across the Welsh Borderland Fault System. Abbreviations are: AL Aymestry Limestone LS Longmyndian Supergroup BG Benton Group LV Llandrindod Volcanics B0 Bentleyford and Orusia Shales MG Menevian Group BV Builth Volcanics MH Milford Haven Group BW Brimmon Wood Member PG Pen y Garnedd Shale CA Caerfai Group PS Pontesford Shales CB Carboniferous Limestone PV Pebidian Volcanic Complex CG Carmel Group RC Ridgeway Conglomerate CH Camlo Hill Group SG Solva Group CL Comley Limestones SK Skrinkle Sandstone Group CM Coal Measures SL Sholeshook Limestone CO Cosheston Group SR Slade and Redhill Beds CS Comley Sandstones SS Shineton Shales CV Castle Vale Formation ST Stapeley Volcanic Group DB Denbigh Grits Group SV Skomer Volcanic Group DG Downton Group TB Trecoed Beds FG Farlow Group UG Uriconian Group FS Folly Sandstone WH Whittery & Hagley Groups GS Grey Sandstone Group WL Wenlock Limestone JG Johnston Group WQ Wrekin Quartzite LF Lingula Flags

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l ACADIAN UNCONFORMITY

400

450

500

Ma

b b' 5 Frs 2 Giv ACADIAN UNCONF 0 Eit > Ems W cl 1400Sig

OF ZMITYl- Ash

i; L

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was transtensional or transcurrentrather than markedly without the need for any relative displacement. Correlation transpressive. of the Edgton and Wenlock Limestones across the Church Reactivation of the Pontesford Lineamentduring the Stretton Faultprecludes any major Acadian and later Acadian event is demonstrated by folding in Silurian rocks displacement on it. along the Clun Forest Disturbance, between the Builth and Duringearly Ludlow time carbonate debris was Shelve Inliers (Woodcock 19846). Some of the Ashgill faults redistributedalong the platformand slope, probably by werere-used, making diagnosis of their earlierkinematic storm-forced flows (Tyler & Woodcock 1987) rather than by history problematic. high-density turbidity flows (Bailey 1969). The shelf to basin slope may not have been marked, but unequivocal axially derived turbidites only occur close to the Tywi Lineament (Fig. 2a, b) as in earlier Silurian time.This suggests a Late Ashgill, Silurian and Early Devonian linkage persistence of similar basin-down components along the The differentiation of the Welsh Basin andthe Midland WBFS. There is a gradational facies change from basin to Platform is again apparent in latest Ordovician and Silurian platform across the WBFS (e.g. Holland & Lawson 1963) sequences. However the contrasts are rather different from that precludes large lateral displacements between the two those of earlierOrdovician time,and a similar type of elements.However major slumping in mid Ludlow time tectonic control cannot be assumed. testifies to continuing tectonic activity along the fault system The Silurian platformsequences are shallow marine (Bailey 1969; Woodcock 1976) and inparticular to a clastics and carbonates with less prominent unconformities possible reversal of palaeoslope along the Tywi Lineament as compared with underlyingOrdovician sequences,and thatheralded the inversion of the whole basin (Tyler & only thin bentonites recording distant volcanism. The basin Woodcock 1987). There was anintermittent shallowing sequencescomprise hemipelagic andturbidite clastics across both the platform and basin in late Silurian time, a lacking volcanics. Faciestransitional between shelf and change to more uniform marine facies andthen a basin are better developed and preserved in Silurian than in conformable transition to continental OldRed Sandstone Ordovician rocks and usually comprise a mud blanket with facies of Pridoli and EarlyDevonian age (Holland & frequent slumps. Lawson 1963). Bothtransects aa‘and bb’(Fig. 2) show thatthe sequences above the Ashgill unconformity form a coherent facies variation from shallow to deep marine, until onset of Acadian tectonics continental conditions in late Silurian time. These rocks are All thethree constituentlineaments of the WBFS were folded and cut by faults along the WBFS, but there is no reactivated in Devoniantime. The Church Stretton requirement for large lateral displacement. Lineament was dominated by faulting with only weak From late Ashgill to mid-Llandovery time,the main bordering folds (e.g. Kirk 1952). A sinistral strike-slip fault clastic supply to the basin was laterally from the platform to system along the lineamentin the Old Radnor Inlier is theSE (Cave1979). A variety of proximal turbiditeand probably of this age (Woodcock 1989) and is compatible mass flow facies close tothe Tywi Lineament (Kelling & with strike-slip evidence from further northeast (e.g. Pocock Woollands 1969; Cave 1979; Smallwood 1986) suggest a et al. 1938; Greig et al. 1968). However, stratigraphic slope or base-of-slope setting that was persistentthrough similarities across the Church Stretton Lineament preclude time and that has not been much dissected by later tectonic large Acadian strike-slip displacements. displacements. The Pontesford and Tywi strands of the Reactivatedbasement faults along the Pontesford WBFS were probably active at this time with a component Lineament propagated up into a belt of tighter folding in the of basis-down dip-slip, although James (1983) argued for Clun ForestDisturbance which shows a flower structure slopecontrol mainly by depositional ratherthan tectonic geometry compatible with limited strike-slip displacement, processes. Zones of thickersedimentation that may of uncertain sense (Woodcock 1984b). Further south-west, represent persistentNW-trending channels or lobes occur basement faults of the Pontesford Lineament may underlie NW of the Tywi line (e.g. Davies 1933; Smallwood 1986). In the major SE-verging monocline alongthe Myddfai Steep the Llandovery district these align with lobes of shallower Belt (Woodcock 1987b). The Tywi Lineament suffered marine facies onthe south-eastern side (Smallwood 1986; strongreverse faulting and folding (Smallwood 1986) and Woodcock & Smallwood 1987) implying littlestrike-slip now marks the south-eastern limit of strong cleavage displacement, but elsewhere this linkage is not obvious. development in the Welsh Basin (Dewey 1969). A strike-slip Upper Llandovery and Wenlock turbidite systems in the component along the Tywi Lineament at this time is basin were mainly supplied axially (Smith & Long 1969; unproven. Cummins 1969; Cave 1979; Smith 1987; Dimberline & Relationships across the Myddfai Steep Belt suggest that Woodcock 1987), probably due to tectonic rejuvenation of the fold and fabric formation along the WBFS is coeval with sources in South Wales. The turbidity currents were laterally the main ‘Caledonian’ deformation in Wales, and that this constrained by a slope whose base lay at the TywiISevern may be as late as late Early to Mid-Devonian (Woodcock Valley line (Dimberline & Woodcock 1987; Smith 1987). 1984a, 19876; Jones 1956). Radiometric dates and dates of The backgroundsediment in the basin is commonly a shedding of Welsh detritus further south suggest an Emsian laminated hemipelagic mudstone facies. This persists on to to Eifelian climax to this deformation event, which Soper et the slope to the south-east,where abundant slump structures al. (1987) correlate with the Acadian event in N America. occur (e.g. Jones 1947), and passes gradationally intothe Soper & Hutton (1984) and Soper et al. (1987) arguefor bioturbated shelf muds andcarbonates of the Midland sinistral transpressive tectonics during this event across the Platform (Hurst et al. 1978). Minorlateral supply of main ‘Caledonian’ Belt in Britain,supported by the carbonate rich debris into the basin links it with the shelf clockwise sense of transection of folds by cleavage (Soper

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some Variscan and laterreactivationof WBFS

300

W 400

4 50 a 0

500

Ma

a n

Fig. 3. Space-time summary of relationships across the Welsh Borderland Fault System. Preserved rock unitsare stippled, unconformities blank.

1986). A similar clockwise transection suggests a sinistral Borderland large Acadian displacements are not required to transpressive model for the NE-SW-trending part of Mid fit the observed facies contrasts across the WBFS, which are Wales (Woodcock et al. 1988). The sinistral strike-slip along no greater than would be expected across a fault-influenced the Church Strettonstrand of the WBFS is probablya basin margin. response to the same deformation regime. The timing of the Acadian event is not well constrained within the WBFS itself. In the Welsh Borderland (transects Post-Acadian events aa’and bb’, Fig. 2) the fault system containsdeformed Stratigraphicrelations suggest thatthe WBFS was rocks as young as Gedinnian, and rocks up to Emsian occur tectonically maintainedasa positive area until Late in the bordering Ludlow Anticline and Brown Clee Syncline Carboniferoustime. Early Carboniferous sediments flank structures to the east (House et al. 1977). Fammenian (Late the northern part of the fault system, which was itself only Devonian) sediments form the base of the post-deformation overstepped in late Westphaliantime (Fig. 3).Lithostrat- overstep sequence in the east where they are only weakly igraphic correlations across the WBFS preclude major unconformable, and the fault belt was only overstepped with subsequent displacements. A similar Ielationship is pre- strong unconformity in Late Carboniferous time (north of served, on the south-eastern side of the belt only, further transect aa’, see Fig. 3). In Pembrokeshire(transect cc’, southwest (e.g. transect cc’, Fig. 2). Fig. 2) the Acadian unconformity spansEarly Silurian to The Variscan deformation strongly affected Pembroke- Early Carbonifeous time, though it may be more narrowly shire northwards to just beyond the Johnston Fault. North represented by the Mid-Devonian gap in theOld Red of this Variscan Front someelements of the WBFS may Sandstone sequence south of the accepted position of the trace intothe obliquedisturbance belts of Red Roses, ‘Caledonian’ front. Eastern Cleddau and Haverfordwest (Hancock et al. 1983) Only in Pembrokeshire doesthe mismatch in pre- butstructural correlation south of thefront is uncertain. Acadian facies suggest the possibility of large displacements Owen’s (1974) correlation of the Church Stretton and onthe interveningfaults, particularly theJohnston Fault. Johnston Faults is now in doubt (Hancock et al. 1983). However there is a Variscan thrustcomponent on these Variscan reactivation of somefaults north of the faults (Sanzen-Baker 1972; Hancock et al. 1983; Smallwood Variscan front is well established (Owen 1974, Owen & 1985) and the magnitude and sense of Acadian displace- Weaver 1983). Based on structural dataon the Swansea ments are in doubt.As outlinedalready, in the Welsh Valley andNeath Disturbances, the sub-parallel Church

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Stretton Lineament may have suffered reverse dip-slip and BRENCHLEY,P. J. & NEWALL,G. 1984. LateOrdovician environmental sinistral strike-slip during N-S Variscan shortening (Owen changes and their effect on faunas. In: BRUTON. D.L. (ed.) Aspects of & Weaver 1983). Sinistraloffsets of Upper Carboniferous rhe Ordovician System, Oslo Universitetsforlaget, 65-79. CAVE,R. 1979. Sedimentaryenvironments of the basinal Llandoveryof rocks occur NE of the Church Stretton area (Greig et al. mid-Wales. In: HARRIS, A. L., HOLLAND, C.H. & LEAKE,B. E. (eds) 1968). However, Permo-Triassic rocks are also affected and Caledonides of theBrirish Isles:reviewed. Special Publication of the significant displacementscould have accompanied the Geological Society, London, 8, 517-26. development of the Cheshire and Staffordshire Permo-Trias COPE,J. C. W. 1977. AnEdiacara-type fauna from SouthWales. Natum, 268,624. basins. - & GIBBONS, W. 1987. New evidence for the relative age of the Ercall Granophyre and its bearing on the Precambrian-Cambrian boundary in southern Britain. Geological Journal, 22, 53-60. Conclusions and discussion CUMMINS,W. A. 1969. Patterns of sedimentation in the Silurian rocks of Wales. In: WOOD. A. (ed.) The Precambrian and LowerPalaeozoic The main conclusions of this paper are summarized on Fig. Rockr of Wales, University of Wales Press, Cardiff, 219-37. 3. Much of the demonstrable faulting across the WBFS may DAVIES.K. A. 1933. The geology of the country betweenAbergwesyn be interpreted as episodic reactivation during Silurian and (Breconshire) and Pumpsaint(Carmarthenshire). Quarterly Journal of later time. Marked stratigraphic contrasts across the western the Geological Society, London, 89, 172-201. edge of the system (i.e. Pontesford Lineament)admit, DEAN,W. T. 1964. The geology of the Ordovician and adjacent strata in the southern Caradoc district of Shropshire. Bulletin of rhe British Museum though do not require, large transcurrent fault movements (Natural History) Geology Series, 9. 259-296. in Ashgill orearlier time.Precambrian correlation across -& DINELEY,D. L. 1961. The Ordovician and associated Pre-Cambrian the westernedge of the WBFS cannot be made owing to rocks of thePontesford district, Shropshire. Geological Magazine, 98, lack of basementexposure in the Welsh Basin area, 367-76. DEWEY,J. F. 1%9. Evolution of theAppalachian/Caledonian orogen. although thereare very broad similarities between all Memoir of the American Association of Petroleum Geologists, 222, Precambrian rocks lying SE of the Menai Strait line. 124-9. Major transpressivedisplacements onthe MenaiStrait DIMBERLINE, A.& J. WOODCOCK,N. H. 1987. The southeast margin of the Fault System wereconfined toLate Precambrian/Early Wenlock turbidite system, Mid-Wales. Geological Journal, 22, 61-71. GEORGE,T. N. 1963. Palaeozoic growth of the British Caledonides. In Cambrian times and docked the Monian terranes against the JOHNSTON, M.R. W. & STEWART,F. W. (eds) The British Caledonides, proto-Welsh Basin (Gibbons 1983a, 1987). Thislate Oliver & Boyd. Edinburgh, 1-33. Cadomian (c. 550-540 Ma)terrane amalgamation is the GIBBONS,W. 19830. TheMonian ‘Penmynydd zone of metamorphism’ in event most likely to havecaused any large transcurrent Llyn, North Wales. Geological Journal, 18, 21-41. - 19836. Stratigraphy,subduction and strike-slip faulting in the Mona movements inboard from the Anglesey area, and may have Complex of North Wales-a review. Proceedings of the Geologists’ been responsible forthe initiation of the WBFS. Association. London, 94, 147-63. Alternatively the WBFS could have formed later, during the - 1987. Menai Strait fault system: An early Caledonian terrane boundary extensional foundering of the Welsh Basin, perhaps during in North Wales. Geology, 15, 744-7. lateCambrian/early Ordoviciantime. Strong Silurian and -& GAYER,R. A. 1985. 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Received 4 January 1988; revised typescript accepted 14 June 1988

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