tectonic evolution of the bristol channel borderlands chapter 7

CHAPTER SEVEN

Tectonic Evolution of the Bristol Channel borderlands

POST-VARISCAN EVOLUTION

Page 7-1

7. EXAMPLES OF POST-VARISCAN REACTIVATION IN THE BRISTOL

CHANNEL BORDERLANDS AND THEIR SIGNIFICANCE IN UNDERSTANDING

THE EVOLUTION OF THE BRISTOL CHANNEL BASIN.

7.1 AIMS

The aims of this chapter are (1) to present evidence to show that post-Variscan fault

reactivation consists of at least two events, an early negative inversion and late positive

inversion; and (2) to show that onshore geology can be used as an analogue for the late

history of the Bristol Channel Fault Zone.

7.2 INTRODUCTION

The Late Palaeozoic in North Devon and Cornwall is locally covered by isolated outliers of

Mesozoic and Tertiary strata whilst the Vale of Glamorgan contains inliers of Upper

Palaeozoic rocks. The partial cover of Mesozoic and Tertiary enables a structural

investigation of the reactivation and inversion of Palaeozoic faults during post-Palaeozoic

tectonism further to the work edited by Cooper & Williams (1989). The current investigation

concentrates on the description of examples of faults that transect the regional Palaeozoic-

Mesozoic unconformity and clearly show different senses of movement above and below the

unconformity. Indisputable examples of inversion are however rare in the Bristol Channel

Borderlands and, to obtain further information on possible reactivation events the movement

vectors on sets of Variscan faults are compared with that of similarly trending post-

Palaeozoic faults in adjacent areas. Opposing kinematic histories are taken to represent

indirect evidence for inversion whilst similar kinematic histories suggest either reactivation

or a common post-Palaeozoic origin.

Evidence of reactivation in the Bristol Channel Borderlands assists in delineating the

Mesozoic and Tertiary structural history of the Bristol Channel Basin to a similar degree of

resolution as Chapman (1989) described the evolution of the Western Approaches Basin. In

particular, structures in Mesozoic strata in the region are used as a guide in defining the

possible reactivation history of the Bristol Channel Fault Zone further than the inversion

event identified by Brooks et al. (1988).

An analogous basin which has undergone reactivation is the Wessex Basin. The structural

evolution of the Wessex Basin (Lake & Karner, 1987) and new data presented by Jenkyns &

Senior (1991) suggested that inversion and intra-Mesozoic faulting were common. Similarly,

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Page 7-2

Gutmanis et al. (1991) gave evidence for composite Mesozoic and Recent movement along

the North Somerset Coastal Fault Zone in the Bristol Channel Basin.

The investigation of post-Palaeozoic tectonism is subdivided into three different themes: (1)

brief regional comparisons of the Bristol Channel Basin and onshore veneers with

neighbouring post-Palaeozoic basins such as the Celtic Sea and Wessex Basins; (2) the role

of onshore analogues in the illustration of the structural history of the Bristol Channel Fault

Zone; and (3) details of reactivation and inversion in the Bristol Channel Borderlands.

7.3 INVERSION TECTONICS IN SW BRITAIN

Early studies of post-Palaeozoic tectonism in SW Britain (eg Jones, 1931) neglected the

possibilities of intra-Mesozoic movements. George (1974) stressed this weakness and

postulated both Mid Jurassic and Mid Cretaceous movements. Folds such as the Cowbridge

Anticline, where the amplitude of the Cowbridge Anticline in the Rhaetic and Lias is about

70m, were taken as examples of structures modified by Mesozoic tectonism. Another

example, the Penmark Syncline, (Chapter 5) is also expressed in Liassic strata of the Vale of

Glamorgan. An early inference on structures in Jurassic rocks was that the Alpine Orogeny

occupied a prolonged interval from Mid Mesozoic times onwards which was followed by a

Mid Caenozoic revival. However comparisons made between the tectonic history of the

Wessex Basin, Celtic Sea Basin and Bristol Channel Basin (Fig. 7.1) show that post-

Palaeozoic deformation is polyphase. Geological surveys along the coasts of North Devon

and South Wales also provide evidence for multiple episodes of negative and positive

inversion beginning with the reactivation of Variscan faults.

Fig. 7.1 Comparison of the tectonic history of the Bristol Channel Basin, the Wessex Basin and

Celtic Sea Basin. Based on Lake & Karner (1987).

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Page 7-3

The following examples of faults evidence several episodes of regional Mesozoic

deformation during the Pre-Alpine history of SW Britain (refer to Fig. 7.2 for localities): (1)

inversion of Variscan faults in the South Wales Coalfield, Culm and North Devon Basins, of

unknown age, eg Moel Gilau Fault; (2) Permian negative inversion at Portledge and

Crediton; (3) Triassic negative inversion at Barry; (4) Liassic synsedimentary faulting at

Penarth; (5) post-Liassic negative and positive inversion of the North Somerset Coastal Fault

Zone (NSCFZ) at Watchet; and (6) Early Cretaceous negative inversion of the Bristol

Channel Fault Zone. The list of faults above illustrates that the pre-Alpine history of SW

Britain was multiphase.

Fig. 7.2 Location of examples of reactivated faults in the Bristol Channel Borderlands. Key: MG

Moel Gilau Fault; D-St M Dinas St Mary's Well Bay Fault; MM Merthyr Mawr Fault; TyW Trwyn-

y-Witch Fault; NP faults at Nash Point; BCFZ Bristol Channel Fault Zone; C Cothelstone Fault;

NSCFZ North Somerset Coastal Fault Zone; LF Lynton Fault; CM Combe Martin Valley Fault; S

Sticklepath Fault; P Portledge Fault; SMM faulting at Speke's Mill Mouth.

The following examples of faults illustrate numerous episodes of possible syn-Alpine Late

Mesozoic and Tertiary reactivation (Fig. 7.2): (1) the E-W trending post-Liassic Trwyn-y-

Witch Fault at Southerndown; (2) the NW-SE trending post-Triassic St Mary's Well Bay

Fault; (3) NW-SE and N-S trending post-Liassic faults at Nash Point and the related Merthyr

Mawr Fault; (4) the Cothelstone Fault at Watchet; (5) the Early-Mid Tertiary Sticklepath

Fault; (6) the Combe Martin Valley Fault. These six faults illustrate that Alpine compression

was also multiphase in SW Britain.

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Page 7-4

7.3.1 ESSENTIAL ELEMENTS IN THE IDENTIFICATION OF A NEGATIVELY

INVERTED FAULT.

Ideally, rocks above the Late Palaeozoic unconformity must be bound by extensional faults

and linked below the unconformity to Variscan thrusts and thrust-related folds.

The three following observations led to the identification of the post-Variscan extensional

faults formed by negative inversion: (1) faulted Permian outliers in North Devon, eg

Crediton and Portledge above folded Carboniferous rocks; (2) areas of faulted, red-stained

Carboniferous rocks of the Culm Basin containing Variscan folds; and (3) faulted Devonian

rocks of the North Devon Basin overlain by red-stained clasts.

Ferric staining of clasts and Upper Palaeozoic rocks in addition to the preservation of local

Permian outliers only within fault-related troughs indicates that the present upland Exmoor

land surface of Devon generally corresponds to the Late Palaeozoic erosion surface and plane

of unconformity (Edmonds et al, 1985). From the observations, above, extensional faults

transect this surface and link with compressional structures below. Therefore there is good

evidence for negative inversion.

7.3.2 EXAMPLES OF EARLY NEGATIVE INVERSION

A likely example of a negatively inverted fault is the Portledge Fault at Peppercombe which

defines the northern margin of the Peppercombe outlier. The Portledge Fault extends

eastwards into Carboniferous strata and has a strike which is identical to the underlying

Variscan structure. The decametre extensional displacement indicates that the Portledge

Fault has a substantial length and probably continues at depth, linking with a Variscan thrust.

However it is also possible that the strike of the Portledge Fault is parallel to an underlying

Variscan fold axis and that it does not link with a thrust. Extensional faulting along axial

planes of chevron folds has been observed, for example, at Hartland Quay. The displacement

on such faults is small in comparison to that on the Portledge Fault. It is therefore likely the

Portledge Fault had a Variscan origin.

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Page 7-5

Fig. 7.3 Photographs A-C show the structural and stratigraphic contacts bounding the Peppercombe

outlier. A, shows a poorly exposed extensional fault juxtaposing Permo-Triassic breccio-

conglomerates and sandstones, and red stained sandstones, siltstones and shales of the Carboniferous

Bude Formation. B & C, show the gentle to moderate dip of the unconformity between Permo-

Triassic and Carboniferous and the reorientation of a Variscan fold immediately beneath the erosion

surface. D, shows the typical Variscan folding and mesoscale thrusting in the red stained Bude

Formation nearby which has an original Variscan orientation.

Fig. 7.4 The grainsize and texture of the red beds in the Peppercombe outlier.

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Page 7-6

The southern margin is marked by a moderately north dipping unconformity between red

breccias and ferric stained sandstones of the Bude Formation (Fig. 7.3). The outlier consists

of ferric coarse to fine grained lithic quartz wacke and monomict pebble breccia containing

clasts of Carboniferous sandstone (Fig. 7.4). Palaeomagnetic data from the breccia and

wacke indicate a Permo-Triassic remnant magnetism (Fig. 7.5). The ferric stained Bude

Formation immediately beneath the unconformity also yields a Permo-Triassic magnetism.

The beds dip moderately and thicken towards the Portledge Fault suggesting (1)

synsedimentary extension and (2) that the Peppercombe outlier is a trap door basin.

Holloway & Chadwick (1986) gave a Permo-Triassic age for extension along the nearby

Crediton Fault that probably represents the same regional phase of negative inversion.

Fig. 7.5 Palaeomagnetic data from the Peppercombe outlier showing a Permo-Triassic

orientation. The axis of magnetisation plunges too steeply to be Carboniferous in age.

Other candidates for Permian inversion are (a) extensional faults displacing ferric

stained chevron folds immediately south of Hartland Quay at Speke's Mill Mouth; and (b)

mesoscale extensional faults along the Foreland Point-Lynmouth section which show drag

close to thrust planes with a contradictory extensional sense of shear (Fig. 7.6). However, in

these examples there are no post-Variscan strata preserved so that the age of negative

inversion is unknown and it is possible that the later negative inversion events described

below produced these structures. NB a similar problem arises in assigning a movement

history to the Moel Gilau Fault in the South Wales Coalfield.

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Page 7-7

Fig. 7.6 A & B Mesoscale displacement thrust from the Foreland Point-Lynmouth section

showing an anomalous drag close to the thrust plane possibly due to post-Variscan extension.

Indirect evidence for Mid Permian inversion is the inferred change in thickness of the

Permian sequence, over 100m thick, underlying the Glastonbury Syncline (Burton Row

borehole, 1:50,000 series sheet 279 & parts of 263 & 295, Weston super Mare). The Permian

is shown to thicken towards faults bounding the Glastonbury Syncline. If these faults are

linked to Variscan thrusts then negative inversion has occurred. There is good evidence for

thrusting at depth eg (Williams & Chapman, 1986; Donato, 1988)

The regional post-Variscan unconformity is exposed along coastal sections in the

west of the Vale of Glamorgan. Triassic conglomerates and Liassic cherty-conglomeratic

limestones onlap Carboniferous Limestone eg at Barry and Trwyn-y-Witch, Southerndown.

At Friar's Point near Barry, south dipping bedding surfaces or thrust flats in Carboniferous

Limestone have been inverted as extensional faults within the overlying breccio-

conglomerates (see section 7.4.5). This is small-scale evidence for Triassic negative

inversion. Observations indicating the expected regional Permo-Triassic negative inversion

are (1) the anomalously thick Triassic sequence (550m) beneath the Glastonbury Syncline

which contrasts with the thin sequence near Wick St Lawrence, north of Weston super Mare;

and (2) the sedimentary variation within the Triassic Mercia Mudstone Group in the western

part of the Wessex Basin (Ruffell, 1990).

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Page 7-8

7.3.3 EXAMPLES OF INTRA-JURASSIC FAULTING

Jenkyns and Senior (1991) gave evidence for intra-Jurassic faulting in the Wessex Basin and

its margins. This faulting is probably related to basement fault reactivation.

There is also evidence for intra-Jurassic faulting in the Bristol Channel Borderlands. The

Rhaetic-Liassic strata of Penarth Head contain moderately south dipping extensional faults

restricted to the Liassic (Fig. 7.7). The faults are truncated above by further Liassic strata and

lose displacement downwards apparently along a bedding surface. These faults represent

evidence for the continuation of local Mesozoic extension and do not represent regional

reactivation.

Fig. 7.7 A-E Examples of tectonic and synsedimentary faults in Rhaetic to Liassic strata from

Penarth Head. The photographs show good evidence for intra-Jurassic faulting and unrelated Late

Mesozoic tectonism. C & D, show views of the same fault. It is unclear, due to erosion, whether it

represents another synsedimentary structure.

7.3.4 LATE NEGATIVE INVERSION EVENT

Brooks et al (1988) gave evidence for the negative inversion of the Bristol Channel Thrust to

form the Bristol Channel Fault Zone. It is shown in Chapter 6 that the Bristol Channel Fault

Zone consists of at least two faults, the Gravel Margin Fault and the Bristol Channel Fault,

which partly or wholly grew in post-Late Jurassic times (probably Early Cretaceous) from the

BCT and GMT (Miliorizos, 1991) (Fig. 7.8). The unconformity between Jurassic-Lower

Cretaceous and Upper Cretaceous strata in the Wessex Basin, South Celtic Sea and Outer

Bristol Channel suggests a pre-Late Cretaceous age for negative inversion of the Variscan

CHAPTER SEVEN



Page 7-9

thrusts beneath the Inner Bristol Channel. A Late Mesozoic unconformity is imaged on

seismic section SWAT 4 Figure 7.9. This section traverses the North and South Celtic Sea

Basins and shows that a major syncline, associated with an extensional fault zone above a

possible thrust, is best developed beneath the Upper Cretaceous layer. Since synclinal folding

is likely to result from displacement gradients and/or the listric shape of faulting, the fault

zone must also have a major pre-Late Cretaceous movement history (probably Early

Cretaceous).

Fig. 7.8 The seismic structure of the Bristol Channel Fault Zone (BCFZ) on Merlin GECO-

PRAKLA line 155 in the Inner Bristol Channel. The interpreted section shows the negative inversion

of the Variscan BCT and GMT which formed the BCF and GMF and splays of the BCFZ.

Fig. 7.9 A part of seismic line SWAT 4 traversing the North and South Celtic Sea Basins. The

section shows the unconformity at Cretaceous levels (highlighted by arrows) overlying a major

syncline above a possible Variscan thrust.

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Page 7-10

7.4 CASE STUDIES OF FAULTS: LATE MESOZOIC AND TERTIARY POSITIVE

INVERSION AND LATE STRIKE-SLIP.

The following section gives field details of faults which may be related to inversion of

Variscan basement faults. The Variscan faults are inverted root faults, defined here as faults

or fault zones that have grown upwards into overlying strata as stem faults which display a

different sense of movement. For example there is good evidence that the Cothelstone Fault

has a Variscan dextral strike-slip component. However at Nash Point where the offshore and

upward continuation of the Cothelstone Fault is predicted to affect the Mesozoic strata there

is evidence for oblique slip and normal faulting. In this case the Variscan basement structure

is the root fault and the Nash Point faults are stem faults. Evidence for inversion is based on

regional considerations of, for example, fault length and displacement.

7.4.1 FAULTS AT NASH POINT

Decametre-length faults displace Liassic strata between Nash Point and Cwm Nash (Fig.

7.10). Faults generally strike NW-SE and N-S (Fig. 7.11) (some trend E-W) and contain

moderate-steeply plunging dip-slip lineations. The sense of movement is oblique and normal.

Displacement is on a decimetre to metre scale (Fig. 7.12). E-W trending faults display

oblique slip displacements. Planar tension gashes strike NNW-SSE and are oblique to the

fault strike.

Fig. 7.10 Stem faults at Nash Point, above the basement related Cothelstone-Merthyr Mawr root

fault, which were probably formed during a Late Mesozoic-Tertiary N-S compressional event.

CHAPTER SEVEN



Page 7-11

Fig. 7.11 Aerial and cliff section views of faulting in the foreshore between Nash Point and Cwm

Nash. Displacements along the faults are on a metre scale. Displacements and movement vectors

along faults at Nash Point contrast with those along the Cothelstone Fault but are probably

kinematically related to the root fault. The change in strike and sense of movement may be due to a

refraction of the principal stresses on passing up sequence from a thick Palaeozoic sequence to a

Mesozoic veneer during the same tectonic event.

7.4.2 THE MERTHYR MAWR FAULT

The NW-SE trending Merthyr Mawr Fault juxtaposes Triassic breccio-conglomerate to the

west and Carboniferous Friar's Point Limestone to the east (Fig. 7.13). The fault strikes 150°

and has an exposed length of 500m across the wave cut platform of Black Rocks, 1km SE of

Newton. Beds in the Friar's Point Limestone are folded close to the fault trace. Clasts in the

breccio-conglomerate display a fracture cleavage that trends about 120° and dips steeply

towards the NE. Minor faults in the Friar's Point Limestone are sub-parallel to the Merthyr

Mawr Fault and show dextral displacements. Mesoscale faults in the breccio-conglomerate

trend 035°. Near Newton Point, on the west side of the Merthyr Mawr Fault, a veneer of

Trias overlies Carboniferous Oxwich Head Limestone. The displacement along the Merthyr

Mawr Fault must therefore be large enough to juxtapose Friar's Point Limestone and Oxwich

Head Limestone. By analogy with the NW-SE trending mesoscale faults in the Friar's Point

Limestone the displacement along the Merthyr Mawr Fault is dextral but the amount of

displacement however is indeterminate.

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Page 7-12

Fig. 7.12 Oblique-slip and normal faulting in Liassic strata of Nash Point.

Fig. 7.13 Sketch map of the Merthyr Mawr Fault at Black Rocks near Newton based on 1:10,000

scale mapping. Key: Carboniferous, FPL Friar's Point Limestone, OxHL Oxwich Head Limestone;

TBr Triassic Breccio-conglomerate.

CHAPTER SEVEN



-13

Fig. 7.14 Structure of the Cothelstone Fault and the North Somerset Coastal Fault Zone near

Watchet.

7.4.3 THE COTHELSTONE FAULT

The Cothelstone Fault crops out at Watchet, Daw's Castle Enclosure and extends across the

wave cut platform to the NW into Warren Bay (Fig. 7.14A). It juxtaposes a variety of

Triassic and Liassic strata along its strike such as (1) the Rhaetic and the White Lias; (2)

thickly bedded Triassic marls and sandstones and thinly bedded buff red Triassic marls; and

(3) Triassic marls and Liassic planorbis beds (Fig. 7.15). The Cothelstone Fault trends about

320° but is linked to the North Somerset Coastal Fault Zone which trends approximately E-

W. At Daw's Castle Enclosure the Cothelstone Fault dips 60° towards the SW. Liassic strata

bounded by splays of the North Somerset Coastal Fault Zone on either side of the

Cothelstone Fault have been offset by about 100m. Related drag folds near the Cothelstone

Fault extend for about 200m from the point of intersection of the Liassic strata and the

Cothelstone Fault; they indicate a post-Liassic dextral strike-slip component (Fig. 7.16).

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Page 7-14

Fig. 7.15 Fault map of the Cothelstone Fault at Daw's Castle Enclosure and the wave cut

platform bordering Warren Bay. The map is based on 1:10,000 scale mapping. Key: 1 & 2

represent normal and late reverse movement along the NSCFZ respectively. Boxes mark

hangingwall of faults; ticks mark side of downthrow.

Fig. 7.16 Sketch map of the drag folding associated with the Cothelstone Fault indicating a

dextral displacement. Key to stratigraphy: T Triassic red marls; Rh Rhaetic green and red

marls; L Liassic limestones and shales. Numbers represent angle of dip.

CHAPTER SEVEN



Page 7-15

Inferences

If the Cothelstone Fault extends north-westwards, it is likely that it is associated with faults

at Nash Point and probably with the Merthyr Mawr Fault. Both the Merthyr Mawr and

Cothelstone Faults have dextral displacements and NW-SE trends. The Nash Point Faults

could be splays to this fault zone formed in a N-S compressive regime in which NW-SE

trending faults moved dextrally whilst N-S trending faults were extensional. Tension gashes,

though oblique to faulting could be kinematically related.

Stratigraphic surveys were carried out on Tusker Rock by A. Ramsay and onshore (eg

BGS Beacon's Down Borehole) by the author to establish the exact present dextral offset

along the Cothelstone Fault (Chapter 6). Even though Mesozoic mesostructure and seismic

evidence of Variscan structure point to dextral strike-slip movement, the author failed to find

evidence of displacements large enough to coincide with those calculated for the Quantocks

and offshore seismic data. However it is possible that the Cothelstone Fault loses

displacement northwards before entering the South Wales Coalfield as the extensional

Gardiner's Fault.

7.4.4 FAULTS AT ST MARY'S WELL BAY

Post-Rhaetic faults from the W side of Lavernock Point to W St Mary's Well Bay (Fig. 7.17)

mainly trend between NW-SE and NE-SW. Few trend ENE-WSW. Fault dips are moderate

or steep towards the east and west. Slickenside lineations plunge gently to moderately. Faults

display normal and reverse dip-slip components but generally a dextral strike-slip component

of displacement. ENE-WSW trending faults are generally reverse.

Fig. 7.17 Oblique-slip (dextral strike-slip) mesoscale faults in Triassic strata of St Mary's Well Bay

which may represent stem faults of the reactivated Dinas cross fault of the South Wales Coalfield.

These post-Triassic faults are related to a major fault which juxtaposes the Triassic in the west and

Rhaetic-Liassic strata in the east.

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Page 7-16

Planar tension gashes trend NNW-SSE and ENE-WSW. In one instance a NNW trending

tension gash displaces an ENE trending tension gash.

Nearby in Sully Bay, there are examples of NE-SW trending envelopes of en echelon tension

gashes indicating sinistral shear and N-S trending planar tension gashes. Decametre length

faults also strike N-S whilst metre-length faults can also strike NW-SE and NE-SW. An

example of a NE-SW trending fault has a sinistral sense of movement. N-S trending faults

have normal displacements. Faults striking E of N have sinistral strike components.

Conjugate joint sets strike NE-SW and NW-SE. Other joints strike E-W and NNE-SSW.

Inferences

The faults at St Mary's Well Bay form a very local fault zone in a section of coast containing

few major faults. It is likely that, similarly to the Cothelstone Fault, the St Mary's Well Bay

faults are linked to a Variscan root structure, the Dinas cross fault, which can be found

further NW within the South Wales Coalfield (Fig. 7.18). The kinematics of the faults and

tension gashes suggest that a N-S compressive regime was dominant, indicating a late

Mesozoic or possibly Caenozoic reactivation of the Variscan cross fault.

Fig. 7.18 A & B Sketch maps of the St

Mary's Well Bay Fault and Dinas cross fault

to the NW. The Dinas root fault has

probably grown upwards into the Triassic

strata of St Mary's Well Bay as a strike slip

fault. Key to (A): P Pontypridd, C Cardiff.

Pz Palaeozoic; Mz Mesozoic; UC

Unconformity. Key to (B): MG Moel Gilau

Fault; TyN Ty'n-y-nant Fault; D Dinas Fault;

C Cymmer Fault; Ll Llanwonno Fault; M

Miskin Fault. Thrusting is represented by

lines marked with triangles on the

hangingwall blocks. The location of (B)

from Woodland & Evans (1964) is shown on

(A).

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Page 7-17

7.4.5 FAULTS AT BARRY AND COLD KNAP

West of Barry at Cold Knap, there are post-Liassic NE-SW and NW-SE and also NNW and

NNE striking faults (7.19). Slickenside lineations are horizontal or plunge gently. NE-SW

trending faults have sinistral strike components.

Fig. 7.19 Examples of strike-slip faults in Liassic strata of Cold Knap near Barry.

Conjugate tension gashes trend NW-SE and NE-SW whilst WNW trending envelopes of en

echelon tension gashes contain NW striking arrays indicating a sinistral shear along a WNW-

ESE trending axis.

Faults at Barry contrast with those at Cold Knap. Near Friar's Point metre-length, dip-slip

normal faults displace the Triassic red marls. Across the irregular unconformity surface

between weathered Carboniferous Friar's Point Limestone and Triassic breccio-conglomerate

there are further extensional faults. Friar's Point Limestone dips moderately towards the

south and contains discrete decimetre to metre spaced bedding surfaces. One example of a

bedding surface or thrust flat has undergone extensional slip which continues upwards across

the plane of unconformity into the breccio-conglomerate. Within the Triassic unit the fault

has a steep dip and displaces clasts of Carboniferous Limestone by about 10cm with a

normal sense. Close to the intersection of the fault and the plane of unconformity there is fine

angular brecciation of clasts within the breccio-conglomerate. The anomalously fine clast

size and close proximity to the fault plane suggest that this localised brecciation is tectonic

and further evidence for post-Triassic extensional faulting.

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Page 7-18

Inference

The contrast in fault types above is due to the polyphase post-Variscan deformation. Early N-

S extension produced faulting at Barry whilst late N-S compression formed the faults at Cold

Knap.

7.4.6 THE TRWYN-Y-WITCH FAULT

The Trwyn-y-Witch Fault (Fig. 7.20A) strikes approximately ENE-WSW along southern

headland of Dunraven Bay and dips moderately towards the south. In its hangingwall it

contains Carboniferous High Tor Limestone overlain unconformably by Liassic Sutton

Stone. In its footwall are other Lower Liassic Marginal Facies and Porthkerry Formation. At

Carboniferous Limestone levels the Trwyn-y-Witch Fault is a south dipping ramp thrust

containing a north facing hangingwall anticline. The fault extends upwards through the plane

of unconformity into cherty conglomeratic limestone forming the Sutton Stone. At these

Liassic levels the Trwyn-y-Witch fault is a strike slip fault. There are at least two sets of

gently plunging slickenside lineations within a narrow fault zone about 1.5m wide. One set

of crystalline slickenside fibres and grooves closest to the fault wall plunges about 25°

towards 235°. The other set, within a veneer of shale upon the crystalline, brecciated fault

wall plunge about 5° towards 080°. Other kinematic indicators such as microfolds and

brecciation fabrics occur in the fault zone (Fig. 7.21). In the footwall near the fault zone

there are moderately tight folds within the Porthkerry Formation at beach level with axial

traces trending WNW-ESE and NW-SE.

Fig. 7.20 A-D (A) is a photo-mosaic of the Heritage Coast from the south side of Trwyn-y-Witch to

Nash Point. It is likely that faulting along the north side of the headland (Fig. 7.22) links with the

decametre scale folding highlighted in the foreground of the mosaic. B-D show examples of

mesoscale faulting nearby for comparison with the Trwyn-y-Witch Fault.

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Page 7-19

Fig. 7.21 Kinematic indicators along the Trwyn-y-Witch Fault.

Inferences

If these folds are kinematically related to the Trwyn-y-Witch Fault, a sinistral sense of

movement is inferred for the fault. This agrees with (1) the sense of movement along the

crystalline slickenside fibres and (2) an interpretation of the brecciation fabric within the

fault zone as mesoscale duplex structures. In argument against a sinistral movement are (1)

an interpretation of the brecciation fabric as a dextral SC-fabric (Fig. 7.22) and (2) the

dextral sense of movement obtained from the late set of slickenside lineations within the

veneer of shale. A likely explanation for all the structures is that a multiphase post-Liassic

strike-slip movement had reactivated a Variscan ramp thrust.

Fig. 7.22 Alternative

interpretations of the

structural fabric in the

brecciated zone

bordering the Trwyn-

y-Witch Fault. A, B &

C show details of the

brecciated fabric

which is also

represented in

photographs of Fig.

7.21.

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Page 7-20

7.4.7 THE NORTH SOMERSET COASTAL FAULT ZONE (NSCFZ)-WATCHET FAULT

The NSCFZ extends E-W along the coast of Somerset. It has been observed at Warren Farm

west of Watchet, St Audrie's Bay and Kilve. The NSCFZ dips moderately towards the south

and generally contains Trias in its footwall and Lias in its hangingwall Figs 7.14 & 7.23),

indicating that it has a major normal downthrow to the south. However near Watchet at

Warren Bay and Warren Farm the NSCFZ has tightly folded Lias in its hangingwall.

Immediately on the west side of the Cothelstone Fault at Daw's Castle the NSCFZ consists of

four splays. Two of the splays exposed in the foreshore closest to the cliff line can be traced

to the E across the Cothelstone Fault and juxtapose Rhaetic, Lias and Trias red marl (Fig.

7.15). East of Daw's Castle the NSCFZ is prominent within the Triassic red marls (Fig. 7.24).

Fig. 7.25 shows the structure within the Triassic red marls of Penarth to emphasise the

intensity of faulting along the Somerset coast due to the NSCFZ.

Fig. 7.23 Structure of the NSCFZ near St Audrie's Bay.

Inferences

The description above indicates that the NSCFZ had a composite movement history. The

post-Liassic extension may have been associated with negative inversion of the BCT and

GMT to the north whilst the late compressive phase may be Late Mesozoic or Caenozoic and

related to the NW-SE dextral strike-slip phase along, for example, the St Mary's Well Bay

faults, Cothelstone Fault and Cold Knap faults. It is possible that the late history of the

Bristol Channel fault zone is analogous to the NSCFZ onshore.

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Fig. 7.24 Prominent faulting and gypsum remobilisation in the Red Marls near Watchet,

associated with the NSCFZ.

Fig. 7.25 Structure of the Red Marls at Penarth and Barry, contrasting with the intense

deformation in Red Marls near Watchet.

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7.4.8 THE STICKLEPATH FAULT

The NW-SE trending Sticklepath Fault crosses Devon and displaces E-W trending Variscan

structures with a dextral offset. Along its length there are Tertiary pull-apart basins which

indicate a sinistral strike-slip displacement (Holloway & Chadwick, 1986). Within these

basins there are Oligocene clays and lignites such as at Petrockstow and Bovey Tracey (Fig.

7.26). To the NW, along strike, the Tertiary granite of Lundy marks the position of the fault

line.

The Sticklepath Fault crosses the coast of North Devon near Abbotsham. Here, exposures of

grey kaolinitic clay can be found of similar texture to that of the local clay basins further

south (Hecht, pers. comm. 1991). There is also evidence for intense deformation and non

pervasive cleavage formation in the Upper Carboniferous rocks of the Culm Basin nearby

(Fig. 7.27). Both suggest that this area of coast has been affected by movement along the

Sticklepath Fault.

Fig. 7.26 A-D Examples of dipping Eocene-Oligocene kaolinitic clay and lignite beds of the

Petrockstow Basin.

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Fig. 7.27 A-H Evidence for locally intense cleavage formation and ductile deformation in

Carboniferous rocks near Greencliff where the Sticklepath Fault is expected to intersect the

North Devon Coast.

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Page 7-24

Inferences

There is clear evidence that in addition to dextral strike-slip along NW-SE trending faults

and reverse movement along the NSCFZ, Eocene-Oligocene sinistral strike-slip has also

occurred. The age relationship between the sinistral strike-slip event and the regional

compression is uncertain. However Holloway & Chadwick (1986) presented evidence for a

local dextral strike-slip event post-dating the major sinistral movement along the Sticklepath

Fault. This suggests that dextral strike-slip on other NW trending faults may also post-date

the sinistral event.

7.4.9 THE COMBE MARTIN VALLEY FAULT

The Combe Martin Valley Fault extends through the harbour of Combe Martin where the

wave cut platform and cliff section are dominated by Variscan folds and thrusts. Leading

inland from Combe Martin there is a NW-SE trending valley which runs along the Combe

Martin Fault. It is likely that the Combe Martin Valley Fault is a Variscan NW trending fault

which has a post-Variscan movement history.

Strike sections through the southern part of the Inner Bristol Channel reveal a large number

of sub-vertical faults cross cutting the Mesozoic sequence. One of the most prominent of

these faults can be correlated directly with the Combe Martin Valley Fault (Fig. 7.28) i.e. the

SE extrapolation of the seismically imaged fault intersects the North Devon coast at Combe

Martin. If this correlation is correct, the Combe Martin Valley Fault normally displaces the

base of the Mesozoic by about 0.1s TWTT (about 150m). This value is in very close

agreement with vertical displacement estimate given by Edmonds et al (1985) of 152.4m.

Lateral displacements of about 2km are also given. Mesoscale evidence for the Combe

Martin Fault may be the presence of tension gashes directed along a NW trend.

Fig. 7.28 Seismic structure of

a steeply dipping fault which

correlates with the Combe

Martin Valley Fault (CMVF).

The amount of displacement

along the fault is small in the

offshore Mesozoic strata.

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Page 7-25

7.4.10 OFFSHORE FAULT-RELATED TERTIARY BASINS

Evidence for a fault-related Tertiary basin occurs on the southern part of Merlin GECO-

PRAKLA line 131, south of Pembroke and west of Lundy. This indicates that the late

sinistral strike-slip event was a significant episode of inversion in N Devon. Obvious

analogues are the Petrockstow and Bovey Tracey Basins along the Sticklepath Fault.

7.4.11 THE MOEL GILAU FAULT

The Moel Gilau Fault is probably the biggest reactivation structure in the South Wales

Coalfield other than the major Caledonoid Disturbances.

The fault strikes east-west and has a variable southerly dip. Based on interpretations of the

northern ends of the Vale reflection lines (Chapter 5) it dips at about 40° and links with a

Variscan flat thrust at a depth of about 3-4km. It is unclear as to whether the geometry of the

fault is listric as quoted by Frodsham (1990). The displacement cannot be resolved on the

seismic data. However Frodsham (1990) stated that the displacement reaches a maximum of

about 1140m, downthrowing to the south and that there appears to be a rapid lateral loss of

displacement eastwards towards a probable tip at which point some of the displacement is

accommodated by the en echelon Ty'n-y-nant Fault. The Ty'n-y-nant Fault has a displacement

of only 90m.

The general east-west trend of the Moel Gilau Fault indicates that it has a Variscan origin.

By analogy to east-west trending faults of North Devon and SW Dyfed, it may even have a

pre-Variscan history, for example the thickness of Middle Coal Measure strata increases

southwards across the fault (Chapter 4). Gayer & Jones (1989) suggested that the Moel Gilau

Fault controlled the passage of northward vergent thrusts into the Coalfield during Variscan

compression. Seismic evidence for this is inconclusive. However Frodsham (1990) stated

that the fault displaces coalfield cross faults and Variscan folding and therefore suggested

that the main surface extension is post-Variscan in age. Frodsham further suggested that the

parallel nature of the Moel Gilau Fault and the Bristol Channel fault zone may indicate a

Triassic extensional reactivation history for the fault. If this analogy with the Bristol

Channel Fault Zone is correct, a major Early Cretaceous movement event is more likely.

However a broader analogy with the Crediton Fault in North Devon suggests that the total

extensional displacement along the fault is partly Permo-Triassic in age.

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7.5 THE ORIENTATION OF TENSION GASHES IN THE VALE OF GLAMORGAN

Fig. 7.29 shows examples of tension gashes from the Vale of Glamorgan which give further

evidence in addition to faulting (eg from Dunraven Bay Fig. 7.30) and open folding of

Mesozoic strata for N-S compression. N-S planar tension gashes and conjugate NW-SE

dextral and NE-SW sinistral sigmoidal tension gashes are common. However it is difficult to

correlate the palaeo-stress system responsible for fault reactivation and tension gash

formation due to the multiphase nature of deformation eg strike-slip along the E-W Trwyn-y-

Witch Fault and oblique slip along NW-SE faults at Nash Point. Furthermore some tension

gashes strike between NW and NE trending sets and have incongruous senses of shear for a

simple N-S compressive system.

Fig. 7.29 A-H Planar and sigmoidal tension gashes in Liassic strata of the South Wales coast.

The photographs show good evidence for N-S trending planar tension gashes and sigmoidal

tension gash envelopes trending other than N-S.

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Page 7-27

Fig. 7.30 Examples of faults from Dunraven Bay. Striation analysis of these faults is

combined with tension gash orientations to reconstruct palaeo-stress orientations.

7.6 STRIATION ANALYSIS OF POST-VARISCAN FAULTS FROM THE BRISTOL

CHANNEL BORDERLANDS.

Fig. 7.31 shows stereographic projections of fault data from the Vale of Glamorgan and

Somerset. Contoured stereographic projections summarise the data and illustrate the deduced

stress orientations for a small sample of faults rigorously measured for the Right Trihedra

Analysis (Lisle, 1988). Both the low correlation value and anomalous stress orientation

obtained are probably a result of a multigenerational sample of faults as would be expected

across the study area. Further and more detailed surveys are required to (1) resolve the exact

stress orientations and (2) to identify discrete populations of faults rather than type examples

as outlined in this study. Once populations have been identified and the stress system qualified

then the sense of movement along reactivated Variscan faults can easily be predicted and

tested by direct field observations.

CHAPTER SEVEN



Page 7-28

Fig. 7.31 Stereographic projections of fault data from the Bristol Channel Borderlands and striation

analysis contour plots to show the deduced palaeostress orientation for a small population of faults

using the Right Dihedra and Right Trihedra method of analysis (Lisle, 1988). A-H, located on the

sketch map, represent small samples of fault data from each locality. The striation analysis plots give

contours in percentage probability of the occurrence of the maximum principal compressive stress

axis at any given point.

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Page 7-29

7.7 CONCLUSIONS

The Bristol Channel Borderlands contain a variety of post-Variscan faults that are in some

cases demonstrably linked to Variscan root structures. There is clear evidence for a

polyphase Mesozoic-Caenozoic movement history which in some cases involved the

inversion of earlier Mesozoic stem structures.

A provisional version of Fig, 7.31 constructed 1989 to 1990.

In light of the evidence for late compression in the Vale of Glamorgan and Somerset it is

likely that the Bristol Channel Fault Zone underwent positive inversion from Late-

Cretaceous to Tertiary times in addition to Early Cretaceous negative inversion.

~~~~~~~~~~~~~~~~~~~~~~~

Marios Miliorizos

7th February 2008

CHAPTER SEVEN



Page 7-30

REFERENCES

Baker, J.W., 1982. The Precambrian of south-west

Dyfed. In:Geological Excursions in

Dyfed, south-west Wales, (ed.) M.G.

Bassett. Published for the Geologists'

Association, South Wales Group by the

National Museum of Wales, Cardiff.

Brooks, M., 1970. Pre-Llandovery tectonism and the

Malvern structure. Proceedings of the

Geologists' Association, Volume 81, part 2,

pp. 249-268.

Brooks, M., 1992. Discussion on the crustal

evolutionary model for the Variscides of

Ireland and Wales from SWAT seismic

data; reply by B. Le Gall with

collaboration of C. Bois and the ECORS-

SWAT Group. Journal of the Geological

Society of London, Volume 149, Part 4,

p. 681.

Brooks, M., Trayner, P.M. & Trimble, T.J., 1988.

Mesozoic reactivation of Variscan

thrusting in the Bristol Channel area, UK.

Journal of the Geological Society of

London, Volume 145, pp. 439-444.

Chapman, T.J., 1989. The Permian to Cretaceous

structural evolution of the Western

Approaches Basin (Melville Sub-basin),

UK. From Cooper, M.A. & Williams,

G.D. (eds), 1989. Inversion Tectonics.

Geological Society Special Publications

No.44, pp. 177-200.

Cooper, M.A. & Williams, G.D., 1989. (eds.).

Inversion Tectonics. Geological Society

Special Publication, No.44. Blackwell

Scientific Publications, Oxford, London,

Edinburgh, Boston, Melbourne.

Dunham, R.J., 1962. Classification of carbonate

rocks according to depositional texture. In

W.E. Ham (ed.), Classification of

carbonate rocks. Am. Assoc. Petrol. Geol.

Mem. 1, pp. 108-121.

Durrance, E.M. & Laming, D.J.C., 1982. (eds.)

The Geology of Devon, University of

Exeter. Edmonds, E.A., Whittaker, A. &

Williams, B.J., 1985. Geology of the

country around Ilfracombe and

Barnstaple. British Geological Survey.

Memoir for sheet 277 and 293 N.S.

Freshney, E.C., Beer, K.C. & Williams, B.J., 1979.

Geology of the country around

Chumleigh. Memoirs of the Geological

Survey of Great Britain, (Sheet 309 N.S.).

Frodsham, K., 1990. An investigation of

Geological structure within opencast coal

sites in South Wales. Unpublished PhD

Thesis, University of Wales, Cardiff.

Gardiner, P.R.R. & Sheridan, D.J.R., 1981.

Tectonic framework of the Celtic Sea and

adjacent areas with special reference to

the location of the Variscan Front. Journal

of Structural Geology, Volume 3, No. 3,

pp. 317-331.

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Gayer, R.A. & Jones, J., 1989. The Variscan

foreland in South Wales. Proceedings of

the Ussher Society, 9, pp. 177-179.

George, T.N., 1970. South Wales (3rd Edition),

British Regional Geology, HMSO.

George, T.N., 1974. The Cenozoic Evolution of

Wales. In: The Upper Palaeozoic and

Post-Palaeozoic rocks of Wales,

(Ed.) T.R. Owen. University of Wales

Press, Cardiff.

Williams, H., Turner, F.J. & Gilbert, C.M., 1954.

Petrography, San Francisco, Freeman.

Gutmanis, J.C., Hailwood, E.A., Maddock R.H. &

Vita-Finzi, C., 1991.

The use of dating techniques to constrain

the age of fault activity: a case history

from north Somerset, United Kingdom.

Quarterly Journal of Engineering

Geology, 24, pp. 363-374.

Holloway, S. & Chadwick, R.A., 1986. The

Sticklepath-Lustleigh fault zone: Tertiary

sinistral reactivation of a Variscan dextral

strike-slip fault. Journal of the Geological

Society, London, Volume 143, pp. 447-

452.

Jenkyns, H.C. & Senior, J.R., 1991. Geological

evidence for intra-Jurassic faulting in the

Wessex Basin and its margins. Journal of

the Geological Society, London, Volume

148, pp. 245-260.

Jones, O.T., 1931. Some episodes in the geological

history of the Bristol Channel region. Rep.

Brit. Ass., Bristol, 1930, pp. 57-82.

Jones, J.A., 1989. Sedimentation and tectonics in

the eastern part of the South Wales

Coalfield. Unpublished PhD Thesis,

University of Wales, Cardiff.

Jones, J.A., 1991. A mountain front model for the

Variscan deformation of the South Wales

Coalfield. Journal of the Geological Society

of London, Volume 148, Part 5, pp. 881-

891.

Kelling, G., 1974. Upper Carboniferous

Sedimentation in South Wales. In: The

Upper Palaeozoic and Post-Palaeozoic

Rocks of Wales, (ed.) T.R. Owen.

University of Wales Press, Cardiff.

Kelling, G., 1988. Silesian sedimentation and

tectonics in the South Wales Basin: a brief

review. In: Besly, B. & Kelling, G. (eds.),

Sedimentation in a Synorogenic Basin

Complex: the Upper Carboniferous of

North-west Europe. Blackie, Glasgow and

London, pp. 38-42.

Lake, S.D. & Karner, G.D.,1987. The structure and

evolution of the Wessex Basin, southern

England: an example of inversion

tectonics. Tectonophysics, Volume 137,

pp. 347-378.

Le Gall, B., 1991. Crustal evolutionary model for

the Variscides of Ireland and Wales from

SWAT seismic data. Journal of the

Geological Society of London, Volume

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148, pp. 759-774.

Lisle, R.J., 1988. ROMSA: A BASIC Program for

Paleostress Analysis using Fault-Striation

Data. Computers & Geosciences, Volume

14, No.2, pp. 255-259.

Mechie, J. & Brooks, M., 1984. A seismic study of

deep geological structure in the Bristol

Channel area. Geophysical Journal of the

Royal Astronomical Society, 87, pp. 661-

689.

Miliorizos, M., 1991. Sub-Mesozoic stratigraphy

and Variscan structure under the Inner

Bristol Channel. (Abstract). Proceedings

of the Ussher Society, Volume 7, Part 4,

p. 430.

Powell, C.M., 1987. Inversion tectonics in SW

Dyfed. Proceedings of the Geologists'

Association, 98, pp. 193-203.

Ruffell, A., 1990. Stratigraphy and structure of the

Mercia Mudstone Group (Triassic) in the

western part of the Wessex Basin.

Proceedings of the Ussher Society,

Volume 7, pp. 263-267.

Woodland, A.W. & Evans, W.B., 1964. The

Geology of the South Wales Coalfield,

Part IV. The country around Pontypridd

and Maesteg. (Explanation of one-inch

geological sheet 248). Memoirs of the

Geological Survey of Great Britain.

England and Wales. Third Edition,

London, HMSO.

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FIGURE CAPTIONS

Fig. 7.1 Comparison of the tectonic history of the Bristol Channel Basin, the Wessex Basin and Celtic Sea

Basin. Based on Lake & Karner (1987).

Fig. 7.2 Location of examples of reactivated faults in the Bristol Channel Borderlands. Key: MG Moel Gilau

Fault; D-St M Dinas St Mary's Well Bay Fault; MM Merthyr Mawr Fault; TyW Trwyn-y-Witch Fault; NP

faults at Nash Point; BCFZ Bristol Channel Fault Zone; C Cothelstone Fault; NSCFZ North Somerset Coastal

Fault Zone; LF Lynton Fault; CM Combe Martin Valley Fault; S Sticklepath Fault; P Portledge Fault; SMM

faulting at Speke's Mill Mouth.

Fig. 7.3 Photographs A-C show the structural and stratigraphic contacts bounding the Peppercombe outlier. A,

shows a poorly exposed extensional fault juxtaposing Permo-Triassic breccio-conglomerates and sandstones,

and red stained sandstones, siltstones and shales of the Carboniferous Bude Formation. B & C, show the gentle

to moderate dip of the unconformity between Permo-Triassic and Carboniferous and the reorientation of a

Variscan fold immediately beneath the erosion surface. D, shows the typical Variscan folding and mesoscale

thrusting in the red stained Bude Formation nearby which has an original Variscan orientation.

Fig. 7.4 The grainsize and texture of the red beds in the Peppercombe outlier.

Fig. 7.5 Palaeomagnetic data from the Peppercombe outlier showing a Permo-Triassic orientation. The axis of

magnetisation plunges too steeply to be Carboniferous in age.

Fig. 7.6 A & B Mesoscale displacement thrust from the Foreland Point-Lynmouth section showing an

anomalous drag close to the thrust plane possibly due to post-Variscan extension.

Fig. 7.7 A-E Examples of tectonic and synsedimentary faults in Rhaetic to Liassic strata from Penarth Head.

The photographs show good evidence for intra-Jurassic faulting and unrelated Late Mesozoic tectonism. C & D,

show views of the same fault. It is unclear, due to erosion, whether it represents another synsedimentary

structure.

Fig. 7.8 The seismic structure of the Bristol Channel Fault Zone (BCFZ) on Merlin GECO-PRAKLA line 155

in the Inner Bristol Channel. The interpreted section shows the negative inversion of the Variscan BCT and

GMT which formed the BCF and GMF and splays of the BCFZ.

CHAPTER SEVEN



Page 7-34

Fig. 7.9 A part of seismic line SWAT 4 traversing the North and South Celtic Sea Basins. The section shows

the unconformity at Cretaceous levels (highlighted by arrows) overlying a major syncline above a possible

Variscan thrust.

Fig. 7.10 Stem faults at Nash Point, above the basement related Cothelstone-Merthyr Mawr root fault, which

were probably formed during a Late Mesozoic-Tertiary N-S compressional event.

Fig. 7.11 Aerial and cliff section views of faulting in the foreshore between Nash Point and Cwm Nash.

Displacements along the faults are on a metre scale. Displacements and movement vectors along faults at Nash

Point contrast with those along the Cothelstone Fault but are probably kinematically related to the root fault.

The change in strike and sense of movement may be due to a refraction of the principal stresses on passing up

sequence from a thick Palaeozoic sequence to a Mesozoic veneer during the same tectonic event.

Fig. 7.12 Oblique-slip and normal faulting in Liassic strata of Nash Point.

Fig. 7.13 Sketch map of the Merthyr Mawr Fault at Black Rocks near Newton based on 1:10,000 scale

mapping. Key: Carboniferous, FPL Friar's Point Limestone, OxHL Oxwich Head Limestone; TBr Triassic

Breccio-conglomerate.

Fig. 7.14 Structure of the Cothelstone Fault and the North Somerset Coastal Fault Zone near Watchet.

Fig. 7.15 Fault map of the Cothelstone Fault at Daw's Castle Enclosure and the wave cut platform bordering

Warren Bay. The map is based on 1:10 000 scale mapping. Key: 1 & 2 represent normal and late reverse

movement along the NSCFZ respectively. Boxes mark hangingwall of faults; ticks mark side of downthrow.

Fig. 7.16 Sketch map of the drag folding associated with the Cothelstone Fault indicating a dextral

displacement. Key to stratigraphy: T Triassic red marls; Rh Rhaetic green and red marls; L Liassic limestones

and shales. Numbers represent angle of dip.

Fig. 7.17 Oblique-slip (dextral strike-slip) mesoscale faults in Triassic strata of St Mary's Well Bay which may

represent stem faults of the reactivated Dinas cross fault of the South Wales Coalfield. These post-Triassic

faults are related to a major fault which juxtaposes the Triassic in the west and Rhaetic-Liassic strata in the east.

CHAPTER SEVEN



Page 7-35

Fig. 7.18 A & B Sketch maps of the St Mary's Well Bay Fault and Dinas cross fault to the NW. The Dinas root

fault has probably grown upwards into the Triassic strata of St Mary's Well Bay as a strike slip fault. Key to

(A): P Pontypridd, C Cardiff. Pz Palaeozoic; Mz Mesozoic; UC Unconformity. Key to (B): MG Moel Gilau

Fault; TyN Ty'n-y-nant Fault; D Dinas Fault; C Cymmer Fault; Ll Llanwonno Fault; M Miskin Fault. Thrusting

is represented by lines marked with triangles on the hangingwall blocks. The location of (B) from Woodland &

Evans (1964) is shown on (A).

Fig. 7.19 Examples of strike-slip faults in Liassic strata of Cold Knap near Barry.

Fig. 7.20 A-D (A) is a photo-mosaic of the Heritage Coast from the south side of Trwyn-y-Witch to Nash Point.

It is likely that faulting along the north side of the headland (Fig. 7.22) links with the decametre scale folding

highlighted in the foreground of the mosaic. B-D show examples of mesoscale faulting nearby for comparison

with the Trwyn-y-Witch Fault.

Fig. 7.21 Kinematic indicators along the Trwyn-y-Witch Fault.

Fig. 7.22 Alternative interpretations of the structural fabric in the brecciated zone bordering the Trwyn-y-Witch

Fault. A, B & C show details of the brecciated fabric which is also represented in photographs of Fig. 7.21.

Fig. 7.23 Structure of the NSCFZ near St Audrie's Bay.

Fig. 7.24 Prominent faulting and gypsum remobilisation in the Red Marls near Watchet, associated with the

NSCFZ.

Fig. 7.25 Structure of the Red Marls at Penarth and Barry, contrasting with the intense deformation in Red

Marls near Watchet.

Fig. 7.26 A-D Examples of dipping Eocene-Oligocene kaolinitic clay and lignite beds of the Petrockstow Basin.

Fig. 7.27 A-H Evidence for locally intense cleavage formation and ductile deformation in Carboniferous rocks

near Greencliff where the Sticklepath Fault is expected to intersect the North Devon Coast.

Fig. 7.28 Seismic structure of a steeply dipping fault which correlates with the Combe Martin Valley Fault

(CMVF). The amount of displacement along the fault is small in the offshore Mesozoic strata.

CHAPTER SEVEN



Page 7-36

Fig. 7.29 A-H Planar and sigmoidal tension gashes in Liassic strata of the South Wales coast. The photographs

show good evidence for N-S trending planar tension gashes and sigmoidal tension gash envelopes trending

other than N-S.

Fig. 7.30 Examples of faults from Dunraven Bay. Striation analysis of these faults is combined with tension

gash orientations to reconstruct palaeo-stress orientations.

Fig. 7.31 Stereographic projections of fault data from the Bristol Channel Borderlands and striation analysis

contour plots to show the deduced palaeostress orientation for a small population of faults using the Right

Dihedra and Right Trihedra method of analysis (Lisle, 1988). A-H, located on the sketch map, represent small

samples of fault data from each locality. The striation analysis plots give contours in percentage probability of

the occurrence of the maximum principal compressive stress axis at any given point.

Word document: PhD Chapter 7 Seven

M. Miliorizos

7th February 2008

tectonic evolution of the bristol channel borderlands chapter 7

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