tectonic evolution of the bristol channel borderlands chapter 4

CHAPTER FOUR Tectonic Evolution of the Bristol Channel borderlands

SW DYFED, CARMARTHEN, SOUTH WALES COALFIELD

Page 4-0

'Searching for Coal'

'I hung the lamp back in its place as I watched Zorba working. He gave his all in his work, he had nothing else

on his mind, he became one with the earth, with the pick-axe, with the coal.

The hammer, the nails were as if they had become part of his body and he wrestled with the wood, he wrestled

with the roof of the drive which had formed a belly; he wrestled with the whole mountain and he wanted to

take its coal and leave.'

Based on 'The Life and Times of

Alexis Zorbas'

by:

NIKOS KAZANTZAKIS.

CHAPTER FOUR

Tectonic Evolution of the Bristol Channel borderlands


Page 4-1

4. LATE PALAEOZOIC STRUCTURAL EVOLUTION OF SOUTH WALES

4.1 AIMS

The aim of this chapter is to summarise the Late Palaeozoic tectonism of South Wales with

emphasis on the main faulting events that have affected the region. It is intended to show that

reactivation was an essential element in the structural evolution of South Wales in terms of

the style of deformation in a foreland basin.

A brief summary is presented of the documented structural controls on Devonian and

Carboniferous facies transitions in South Wales as a means of identifying the age and

kinematics of the regional reactivation events. Detailed studies are then made of the

composite fault history and style of deformation (1) in the Devonian of Llanstephan, SW

Dyfed (2) in the Carboniferous Limestone of South Wales and (3) within the opencast coal

sites of the South Wales Coalfield (OCCS).

Comparisons are drawn with the reactivation history and structural style of deformation in

North Devon (Chapter 3) and the effects of reactivation within different coalfield structural

environments and on coal distribution are discussed.

4.2 THE PRE-LATE PALAEOZOIC FRAMEWORK OF SOUTH WALES

Introduction

Theoretical considerations of the structural limits to reactivation (Chapter 1) were applied to

the pre-Variscan fault framework of SW Wales. After examining the orientation of pre-

Variscan faults the likelihood of reactivation became evident. In light of the structural trends

observed in SW Dyfed, the origins of coalfield disturbances other than the major Swansea

and Neath Valley Faults could be questioned in terms of Caledonian origins.

A suitable example from SW Dyfed of a fault of pre-Variscan origin is the Carreg Cennen

Fault. It has a Caledonoid trend but also clearly separates the Lower Palaeozoic Welsh Basin

facies of SW Dyfed from the Midland Platform facies of the Welsh Borderlands, where it has

a pre-Caledonian movement history. Late Caledonian uplift of the Welsh Basin must have

been accommodated by the Carreg Cennen Fault since Devonian sediment was shed towards

the south and south-east across the fault. The regional movement history during Devonian

times has been outlined in Chapter 3 and in section 4.3.2 below. It is unclear whether the

Carreg Cennen Fault had a Devonian movement history but during an Early Devonian N-S

extensional regime (Powell, 1987) the Carreg Cennen Fault would be expected to show

CHAPTER FOUR



Page 4-2

an oblique normal sense of movement. Cope (1979) however stated that Devonian strata to

the north of the Carreg Cennen - Llandyfaelog Disturbance have been thrust southwards.

Structural evidence such as this has recently been interpreted by Woodcock (1988) and

Woodcock & Gibbons (1988) to represent Early-Mid Devonian Acadian deformation, i.e. the

entire Devonian may not have involved a continuous extensional phase.

During Dinantian and Namurian times, however, it is likely that thickness variations and

facies transitions from the main coalfield into SW Dyfed (Owen, 1974) were caused by fault

movement (Fig. 4.1).

It is suggested here that the rejuvenation of St. George's Land during Early Namurian times

(Kelling, 1988) was accommodated by the extensional reactivation of the Carreg Cennen

Fault i.e. the superposition of an extensional regime of the Namurian peripheral upwarp on

the pre-existing Early Carboniferous extensional fault template (in contrast to the regional

Late Carboniferous structural setting: section 4.3.2). This would have been the latest phase of

movement prior to the main phase Variscan positive inversion and thrusting recorded in the

Devonian strata of Llanstephan (south of the Carreg Cennen Fault).

(It is likely however that any extension associated with peripheral upwarping is local in

extent and time so that Namurian sedimentation probably records the onset of compressional

tectonism).

Most significantly the Carreg Cennen fault generally subdivides Lower and Upper Palaeozoic

stratigraphy and Caledonian and Variscan structure. The exact subdivision could be

attempted on comparing Devonian geology on either side of the disturbance.

CHAPTER FOUR



Page 4-3

St. David's

Examination of geological maps of the St. David's area (Green, 1908; BGS 1:50 000 sheet

209 St. David's) followed by field investigation revealed a number of pre-Variscan structural

trends: a NE-SW and NNE-SSW fault trend and a predominant ENE-WSW Caledonoid fold

trend (Fig. 4.2).

This trend was investigated as a means of establishing an analogue for 'anomalously' trending

disturbances in the coalfield.

Conjugate tension gashes, found in the Fishguard area, trending NE-SW and NW-SE are

assigned a Variscan age (based on comparisons with the trend of conjugate Variscan faults in

North Devon, Fig. 4.3). It is however uncertain whether these structures are due to pristine

Variscan tectonism. Conversely if NW trending structures are pre-Variscan then other larger

NW trending structures of the coalfield, the cross faults may also have pre-Variscan roots.

CHAPTER FOUR



Page 4-4

This district does contain the pre-Variscan NE-SW Caledonoid trend anticipated to have been

involved during Variscan reactivation of faults within the coalfield. However the local ENE-

WSW trend in SW Dyfed clearly deviates from the regional NE-SW trend of the Welsh

Basin (BGS sheet United Kingdom, south, scale 1:625 000, & Fig. 4.4). The ENE-WSW

trend may either be a primary feature of the Caledonian belt or a modification brought about

by Variscan compression. The lack of evidence for intense straining which would be

expected in a Variscan change of strike involving about 23° clockwise rotation of a

Caledonian terrane suggests that the ENE trend in SW Dyfed is a primary feature of

Caledonian tectonism.

CHAPTER FOUR



Page 4-5

This leads to the interpretation of ENE-WSW and even E-W trending disturbances within the

South Wales Coalfield (as well as the NE-SW trending disturbances) as structures related to

underlying pre-Variscan faults eg the Trimsaran Disturbance trending E-W (Archer, 1968)

and the Llannon Disturbance trending ENE-WSW.

Thus SW Dyfed contains structures which represent candidates for pre-Variscan faults which

are taken to be analogues of 'root faults' for some of the more anomalously trending

disturbances within the coalfield.

Conversely it can be argued that the trend in SW Dyfed is uncharacteristic of the Caledonian

trend and that in fact the regional NE-SW trend of basement structures extends beneath

central South Wales as far as the Usk and Malvern axes and that disturbances in the coalfield

striking other than NE-SW are pristine Variscan structures. Fig. 4.5 illustrates the possible

hypothetical Palaeozoic movement histories of anomalously trending disturbances in relation

to north-westerly disturbances.

CHAPTER FOUR



-6

4.3 REGIONAL EVIDENCE FOR FAULT REACTIVATION DURING THE LATE

PALAEOZOIC HISTORY OF SOUTH WALES.

4.3.1 INTRODUCTION

A synopsis of the coalfield stratigraphy of South Wales is given below. It emphasises

published evidence for synsedimentary fault movement and identifies Late Palaeozoic

reactivation events additional to the main positive inversion of Devonoid faults during

Variscan thrusting (Le Gall, 1991).

Nb the term reactivation, here is also taken to infer the general episodes of renewed

movement along groups of faults within South Wales. In this sense positive basin inversion

can be correlated with regional uplift and may be the mechanism involved.

The three coalfields around South Wales: Forest of Dean & Bristol, Central South Wales, SW

Dyfed are erosional remnants of the Variscan foreland basin, defined in Chapter 2. The South

Wales Coalfield is the largest component outlier of the foreland basin set on the northern

outer arc of the Variscan orogenic belt (Fig. 4.6). It is 95km long and 30km wide and

contains rocks ranging in age from Early to Late Carboniferous (Stephanian).

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Page 4-7

The Carboniferous strata mainly overlie Devonian Old Red Sandstone (ORS) apart from

areas such as SW Dyfed where they overstep older strata (BGS 1:50 000 sheet 228

Haverfordwest) and are overlain by an unconformable veneer of post-Variscan Mesozoic

strata, eg, in the Vale of Glamorgan (BGS 1:50 000 sheet 261/262 Bridgend). It is bordered

to the north-west by the Devonian of the Brecon Beacons (BGS 1:50 000 sheet 213 Brecon),

to the east by the Malvernoid Usk axis deforming mainly Lower Palaeozoic strata (BGS

1:250 000 Bristol Channel sheet 51°N - 04°W), and to the south by the Cowbridge Anticline

which folds outcropping Devonian conglomerates and Carboniferous Limestone of the Vale

of Glamorgan (BGS 1:50 000 sheet 261/262 Bridgend).

The South Wales Coalfield displays excellent examples of Variscan structures on all its

outcropping margins and within the opencast sites. As discussed below, previous studies of

the stratigraphy of the South Wales Coalfield demonstrate that many Variscan faults in the

South Wales Coalfield have composite movement histories.

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Page 4-8

4.3.2 STRATIGRAPHIC EVIDENCE FOR REGIONAL FAULT REACTIVATION

ORS of South Wales

Extensive literature describes the ORS of South Wales. The detailed sedimentology of the

ORS was studied by Ball et al (1961), Allen (1962, 1964, 1965a&b), Pick (1964), Allen &

Tarlo (1963) and the stratigraphy of the ORS was studied by Dixon (1921), Williams (1971),

Sanzen-Baker (1972) in SW Dyfed, by King (1925, 1934) in the Welsh Borderlands, by

Straw (1930, 1953), and Potter & Price (1965) in south-central Wales and by Pringle &

George (1948) who include descriptions on the ORS of the Vale of Glamorgan.

Emphasis is placed here on the contrast in Devonian facies across the Bristol Channel and

especially on Middle Devonian stratigraphy (Fig. 4.7). The regional differences in facies are

attributable to broad palaeoenvironmental patterns, whilst local Mid Devonian facies

transitions are due to fault movement (Chapter 3) which may have involved partial

reactivation of Early Devonian extensional faults.

CHAPTER FOUR



Page 4-9

Clearly the local occurrence of polymict conglomerates in the Vale of Glamorgan, SW Dyfed

and the Mendips and the Middle Devonian unconformity in central South Wales (Fig. 4.8)

are good evidence for the formation and denudation of local uplands in the Bristol Channel

area.

It is suggested here (after Tunbridge, 1986) that the distribution and composition of the

Middle Devonian conglomerates are related to Devonian fault activity in the Bristol Channel

area. However this fault activity is more likely to be associated with extension than with end

Caledonian or early Variscan compression. This is likely because the conglomerates and

Middle Devonian unconformity in South Wales (Fig. 4.8) could have been formed in

response to footwall uplift of South Wales and St. George's Land. (This model assumes little

or no Mid Devonian movement along the Carreg Cennen Fault and classes 'Acadian'

structures of N Llanstephan to be Variscan structures) (Fig. 4.9).

The following cases represent four hypothetical Devonian movement histories for the Carreg

Cennen Fault (Fig. 4.10).

The entire Devonian tectonic history of the Bristol Channel Borderlands may have been set in

an extensional regime. This is in agreement with the character of a passive margin

environment as reflected by the sedimentology of North Devon. This would indicate a large

scale extensional environment since North Devon would have been at a much greater

distance form South Wales prior to Variscan thrusting.

CHAPTER FOUR



Page 4-10

CHAPTER FOUR



Page 4-11

Conversely if the scale of Mid Devonian uplift in South Wales is considered (affecting the

whole of St. George's Land), a separate structural phase could be inferred i.e. Acadian

deformation may have affected pre-existing Caledonoid and Devonoid faults in an unrelated

compressional sense.

Carboniferous Limestone

The stratigraphy of the Carboniferous Limestone has been studied extensively (eg Vaughan,

1905; Dixon & Vaughan, 1911; George, 1950, 1970; Ramsbottom, 1970, 1973, 1979). In

general the Carboniferous Limestone consists of oolitic, skeletal and dolomitic limestones

and undergoes major dramatic thickness variation across the South Wales Coalfield, from

1500m on Gower to 100m on the North and East Crops.

Further local thickness variations have already been attributed to synsedimentary deformation

eg in the Gower area (George, 1927, 1958, 1970) (Ramsay pers. com. 1991). Facies

transitions grade into disconformities across major faults in the coalfield suggesting that

Caledonoid and Malvernoid trending structures were active during the Dinantian eg

(Blundell, 1952). Recent studies by Ramsay (pers. com. 1991) provide detailed stratigraphic

evidence for intra-Dinantian fault-block movements. There is further evidence for

extensional fault-controlled sedimentation from SW Dyfed (Sullivan, 1964) which points to

an extensional movement along east-west trending faults from Late Devonian to Dinantian

times. This would represent a reactivation event rather than an inversion event as long as

movement was not continuous between the different phases of the same senses of movement.

Other detailed studies of eg the Caninia Zone by Sullivan (1964) have shown distinct facies

transitions and thickness changes across the Ritec Fault pointing to Mid Dinantian

extensional movement.

Millstone Grit

Extensive literature exists on the Namurian Millstone Grit and on the unconformable nature

of its contact with the underlying Carboniferous Limestone (eg Dixey & Sibley, 1918; Dixon

& Pringle, 1927; George, 1927, 1956; Evans & Jones, 1929; Robertson & George, 1929;

Ware, 1939; Blundell, 1952; Jones & Owen, 1966).

The Millstone Grit exhibits major changes of thickness and facies across South Wales (eg

Strachan, 1909; Robertson, 1927; Moore, 1945, 1948; George, 1956b). Eg

• Gower shale, marine bands

• East Crop quartz arenite & quartz conglomerate

• North Crop quartz arenite & quartz conglomerate

black shales; marine bands.

CHAPTER FOUR



Page 4-12

Similarly to the Carboniferous Limestone, the Millstone Grit thins regionally to the north-

east. George (1970) used facies transitions and thickness variations to construct the

palaeogeography of the Namurian in light of biostratigraphic studies by Evans & Jones

(1929); Dix (1933); Ware (1939); Moore (1948) and demonstrated the northerly derivation of

Namurian sediment (Fig. 4.11). The lithostratigraphic boundaries within the Millstone Grit

were also found to be diachronous (eg Pringle & George, 1948).

The observations above can be interpreted in terms of a tectonic control on Namurian

sedimentation as well as Dinantian sedimentation but the nature of the causal structural

environment is in question. According to Kelling (1988) it is likely that the onset of

siliciclastic sedimentation in South Wales during the Late Dinantian - Early Namurian was a

response to initial Variscan tectonic loading. It is suggested here that previous extensional

fault controls were due to an unrelated structural environment inherited in South Wales from

Late Devonian times. In agreement with Kelling (1988) a regional positive inversion event is

thought to have occurred during Early Namurian times in which an extensional carbonate

platform was replaced by a compressional siliciclastic basin sourced by a rejuvenated

landmass St. George's Land to the north and east, i.e. the converse and more likely alternative

to the hypothesis stated in section 4.2 on extensional deformation associated with peripheral

upwarping. Namurian sedimentation therefore represent the regional onset of Variscan

compression in South Wales and also a regional inversion event.

CHAPTER FOUR



Page 4-13

Coal Measures

The conformable contact between the Millstone Grit and the succeeding fluviatile

Westphalian Coal Measures was described by Evans & Jones (1929) and the constituent

sequences of the Coal Measures were described by Trueman (1947), Woodland & Evans

(1964), Archer (1968), Squirrel & Downing (1969), George (1970), Thomas (1974).

The conformity between most of the Coal Measures and underlying strata points to a

continuation of a similar tectonic regime from Namurian times as does the fact that the Coal

Measures are also found to thin eastwards towards the Usk axis (Fig. 4.12) (eg Blundell,

1952). The Coal Measures have a maximum thickness of 3000m in the Swansea area and thin

to about 500m in the North Crop and East Crop. Seam splits also occur on passing westwards

from the East Crop which suggests that the same positive tectonic feature to the north and

east of the basin affected both Namurian and Westphalian sedimentation. In agreement with

Kelling (1988), the regional change in thickness of Westphalian sediments is attributed to the

formation of a positive feature to the north and east of the basin and to the enhanced

subsidence in the south in response to Variscan tectonic loading (Fig. 4.13). Furthermore it is

suggested here that the exact locations of the structural rise and the deepest part of the basin

were controlled by basement lineaments such as the Usk Axis, Carreg Cennen Fault and

possibly the basement roots of north-westerly trending cross faults (see section 4.9 for

details).

Thickness variations (Fig. 4.14) in the Upper Coal Measures, the Pennant Measures of

Strachan (1899) and Woodland et al (1957) are distributed similarly to the Millstone Grit and

earlier Coal Measures which suggests that basement faults were active throughout the Late

Carboniferous. However the diachronous onset of Pennant-type sedimentation also reflects

an episode of renewed Variscan thrusting (see Chapter 2) following the deformation of the

Culm Basin (Chapter 3).

It would consequently be expected that the effects of basement fault reactivation would be

shadowed by the main phase thrusting during Pennant sedimentation.

CHAPTER FOUR



Page 4-14

Palaeocurrent data from the Pennant Measures (Jones, 1989, 1991) suggest that early

Variscan folds caused the observed deflections in fluvial current direction from the regional

northerly flow whilst major main-phase thrusts were developing to the south and were

uplifting the South Crop.

In summary, the stratigraphy of the South Wales Coalfield reveals good evidence for a

variety of structural settings during the Late Palaeozoic, with the implication of reactivation

of major regional structures.

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Page 4-15

4.4 A CASE STUDY OF COMPOSITE VARISCAN DEFORMATION IN DEVONIAN

STRATA OF LLANSTEPHAN, SW DYFED.

4.4.1 INTRODUCTION

Llanstephan lies near the intersection of the Carreg Cennen Fault with the South Wales coast

(Fig. 4.15a & b). Along this coastal section there is good exposure of Devonian fluviatile and

lacustrine red sandstones both rich in calcareous deposits. The section is shortened by folds

and thrusts (in a similar style to that observed along the Foreland Point section).These

structures were investigated in relation to the regional trend of the Carreg Cennen Fault.

Furthermore the origin of the early thrusts and late folds was considered in terms of the

regional history of the Bristol Channel Borderlands. It remains partly unsolved as to whether

the early thrust/late fold field relationship represents the effects of firstly reactivating the

Carreg Cennen Fault at depth followed secondly by the generation of new structures at

shallower levels.

Two sets of structures were observed:

1. early thrusts and tension gashes

2. folding and late mesoscale thrusts

CHAPTER FOUR



Page 4-16

4.4.2 EARLY THRUSTS AND TENSION GASHES

Early east-west trending thrusts show decimetre to metre scale displacements and strike

lengths of around 10m. Transport directions are towards the north and south (Fig. 4.16) with

slickenside lineations displaying a down-dip slip vector and plunging at various angles

(gentle-steep) depending on the folded geometry of the thrust planes (Fig. 4.17). East-west

trending tension gash arrays display a dextral shear sense (Fig. 4.18).

CHAPTER FOUR



Page 4-17

The above structures are compatible with sinistral strike-slip reactivation of the Carreg

Cennen Fault (Fig. 4.19).

I.e. the thrusts and tension gashes represent structural components of the strike-slip

movement.

CHAPTER FOUR



Page 4-18

4.4.3 LATE FOLDS AND THRUSTS

The later anticlinal fold has a wavelength of about 3km and an open to moderate profile; it

faces towards the NNW and plunges gently towards the east and west. The axial plane dips

steeply towards the SSE (see Fig. 4.20). The fold rotates the early thrusts about its ENE axis

by up to 50° towards the north and 10° towards the south. The fold is in turn deformed by

gently dipping southward transporting mesoscale thrusts (Fig. 4.21) which are the latest

structures observed along the section.

Nb the interpretation of thrusts as being pre- or post-folding depends on the angle of dip of the

thrust planes in relation to the dip of bedding and the transportation direction of the thrusts in

relation to their structural position on the fold (Fig. 4.22).

CHAPTER FOUR



Page 4-19

4.4.4 INFERENCES

These observations strongly support a bipartite structural evolution of the area. Even if the

early thrusts and tension gashes represent local structural components accommodating early

strike-slip movement along the Carreg Cennen Fault, compartmentalisation of the early

deformation has occurred in a different manner to that in other areas close to disturbances eg

Cribarth in the Swansea Valley and Gilfach Iago near the Caerbryn Disturbance (see below).

The main difference is that the early thrusts do not follow the trend of the Carreg Cennen

Fault to compartmentalise the early deformation in the following manner (Fig. 4.23a & b):

The transport direction of the early thrusts at Llanstephan represents the regional σ1 principal

compressive stress rather than the σn normal stress to the Carreg Cennen Fault during fault

reactivation at depth. The late thrusts are taken to represent pristine Variscan structures

similar to the back thrusts along the south crop of the coalfield and in opencast sites such as

Ffos Las (see section 4.5.5).

The local evidence suggests that back thrusting post dates large scale folding. Either one

progressive phase of deformation can be invoked or the early thrusting and late folding can be

attributed to a fault reactivation event and the later back thrusts to buttressing against

CHAPTER FOUR



Page 4-20

St. George's Land during Variscan deformation unrelated to basement faulting. This latter

interpretation places regional significance on the mesoscale thrusts recognising that

throughout the Bristol Channel Borderlands the early thrust/late fold field relationship is

widespread. This suggests that to include folding of the Llanstephan section as being related

to fault reactivation may not be coherent with the regional structural style (nb a similar

problem arises in the interpretation of Gilfach Iago section 4.5.7).

An obvious beneficial result of this investigation is that the mesoscale structural style

associated with fault reactivation along a major fault has been described.

4.5 VARISCAN STRUCTURES IN THE OPENCAST COAL SITES (OCCS) OF THE

SOUTH WALES COALFIELD.

Overview

Field surveys were undertaken to investigate variations in structural style and relative age of

faults and folds within the opencast coal sites, mainly along the north crop of the South Wales

Coalfield foreland basin.

The surveys identified fault types recognised previously within the basin:

(1) East-west trending strike-thrusts:

(a) early, decametre-scale, folded thrusts

(b) late, southward dipping, northward directed

fore-thrusts with maximum displacements of 1km

(c) northward dipping, southward directed break-back

thrusts.

The dominant thrusting styles are thought to be:

(i) in-sequence, foreland-propagating, leading

imbricate fans

(ii) out-of-sequence, trailing imbricate fans which can cut

down section, northwards, through early in-sequence

sets

(iii) mesoscopic, northward directed duplexes within coal

seams and mudrocks in lower sections of the Middle

Coal Measures (MCM)

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Page 4-21

(iv) mesoscopic, near-planar, gently dipping isolated

thrusts within lower-middle sections of the MCM.

The thrust assemblage is thought to have formed either by

progressive deformation or in relation to the regional events

involving fault reactivation or the generation of new

structures.

(2) North-south and NW-SE trending normal faults termed cross faults.

(3) Contractional faults accommodating folding.

(4) East - west trending normal strike-faults termed lag faults.

Examination of the sites reveals variation in structural style related to geographic and

stratigraphic position. Deformation is greatest in terms of shortening in western sites reaching

its maximum in the anthracite field. Stratigraphically, deformation predominates in the

Westphalian B, within lower-middle sections of the MCM.

The geographic control on structural style across the basin has previously been attributed to

differences in proximity to:

(1) 'Disturbances':

(a) east-west trending zones of intense folding and thrusting

active during Variscan deformation eg, the Trimsaran

Disturbance (Frodsham, 1990).

(b) NE-SW trending zones related to basement faults which

contain normal cross-faults and north-south striking,

eastward directed thrusts (thrust cross faults) eg,

the Swansea Valley and Neath Valley Disturbances

(Owen, 1954).

(2) The southern orogenic hinterland and the north-western cratonic landmass.

The stratigraphic control on structural style is attributable to differences in the rheology of the

Coal Measures (Cole et al, 1991). Particular coals in the lower sections of the MCM,

CHAPTER FOUR



Page 4-22

however controlled the geometry and development of strike-thrusts at various scales in a

similar fashion in different sites across the coalfield.

The relative ages of the structural classes are discernible only within sites. Previous work has

shown that synsedimentary tectonics occurred on east-west trending faults and folds (Jones,

1991). Sedimentary growth sequences across NW-SE oriented normal cross-faults also

indicate active extension during sedimentation (Hartley, pers. com., 1990). However, many

cross-faults post-date strike-thrusts and show no synsedimentary growth features at least in

the MCM. Most strike-thrusts are also entirely post-depositional (Cole et al, 1991).

The surveys clearly reveal the significant structural control on the location and concentration

of coal within the South Wales Coalfield.

4.5.1 LIST OF SURVEYED OPENCAST COAL SITES

The fault history and structural style will be described in each of the following opencast coal

sites of the South Wales Coalfield located in (Fig. 4.24):

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Page 4-23

Opencast coal sites (OCCS):

• Benward's Field

• Derlwyn

• East Pit Extension

• Ffos Las

• Ffyndaff Additional

• Garnant

• Gilfach Iago

• Nant Helen

• Park Slip Extension

• Pen Bryn Oer

Structures at each site are described in terms of their position in relevance to the regional

structural picture.

4.5.2 STRATIGRAPHIC STANDARD FOR THE OPENCAST SITES

The stratigraphy adopted for the opencast coal sites of South Wales is that constructed by Mr

R. Thewlis of British Coal who has correlated coal seams and numbered cyclothemic

sequences across the South Wales Coalfield. Most of the coal seams observed in the opencast

sites occur within the middle part of the MCM between the Amman and Cefn Coed Marine

Bands (the Vanderbeckei and Aegiranum Marine Bands).

Each seam within the opencast sites may contain various types of repetitions which are either

identified on passing up sequence using sequential letters or numbers as suffixes or denoted to

be below by the term Lower or above a given seam by the terms Upper or Rider. Most of the

sites contain seams that have been repeated a number of times by thrusting so that the

apparently simple stratigraphy given for each site is complicated by sub-seam names. The

structure of each seam will be described as a unit for all sites.

nb a correlation of coals of the OCCS is given in Appendix 4.1.

4.5.3 STRUCTURAL HISTORY

Examinations of the Variscan structures in the opencast coal sites of the South Wales

Coalfield has revealed a structural history more complex than those previously described (eg,

Jones, 1989). The general structure is however found to be in agreement with the evolution

proposed by Jones (op cit), namely: 1. extension; 2. early folding; 3. main compression; 4.

extension.

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Page 4-24

This structural history is investigated below:

1. There is evidence for early synsedimentary extension in a number of sites. Non-tectonic

synsedimentary extension has been previously described eg (Hartley & Gillespie, 1990; Cole

et al, 1991). The present study cites new evidence from the coalfield for early tectonic

extension.

2. & 3. Previous work by Jones (1989) has demonstrated an early folding event and a main

compressional event. The present study includes observations of early thrusts and folds

deformed by major folding as evidence for two compression phases.

4. The late cross faulting, characteristic of the South Wales Coalfield (and North Devon),

represents a late extensional phase in South Wales. New examples of this phase are cited from

the opencast sites.

It is important to note that Jones (1989) basis for the bipartite compression (2 & 3) is different

from that of the present study. As stated in Section 4.3.2 (Coal Measures) Jones used

palaeocurrent data from the Pennant Measures as evidence for the early folding event (2)

(which may have been related to basement fault reactivation) and deep mine and opencast

data for the late compressional event (3). The present study provides direct structural evidence

from the opencast sites for the early thrust/late fold field observation to demonstrate that the

early compressional event (2) was overprinted by a late main compressional event (3) as

above.

The same question arises, as in Llanstephan and North Devon, as to whether the early

structures equate to fault reactivation and the late structures to the onset of Variscan thrusting

unrelated to old faults. The present study shows that some faults eg cross faults, had long

movement histories which spanned the various structural episodes from 1 to 4. It needs to be

considered whether such faults provide evidence that the compressional deformation in the

South Wales Coalfield was progressive and in the style of PEST (Frodsham, 1990) or

conversely, that it was composite and dominated by reactivation events (this study).

Observation of the structures, especially in the western part of the coalfield where shortening

is greatest, shows that deformation close to disturbances produced cross cutting relationships

far more numerous than would a simple bipartite compression, eg Llanstephan and Cribarth.

Each opencast site is described in detail with emphasis being placed on observations

supporting the structural evolution outlined above (Fig. 4.25).

CHAPTER FOUR



Page 4-25

Inferences on the regional structure are also made by drawing comparisons with adjacent

areas eg, Gower, Vale of Glamorgan, SW Dyfed.

4.5.4 PARK SLIP EXTENSION

Overview of a mature site

Park Slip Extension, which has now been back-filled, showed in its later stages of

development a gentle to moderate dip of bedding corresponding to the regional northerly dip

of the south crop of the coalfield. Investigations were made into the structural history of the

site and the origin of the regional moderate northerly dip of the south crop of the South Wales

Coalfield.

The east-west striking northern high wall, buried by back-filling during the winter of 1989,

showed a horizontal dip of strata due to the section being sub parallel to the strike, with no

distinct cross faulting. In uppermost eastern parts of the high wall, metre scale southward

directed thrusts were observed in oblique section. In contrast the south-eastern benched wall

contained beds dipping locally at 50° towards the north. Cross faulting was observed during

visits prior to the present study (Gayer pers. com. 1990).

CHAPTER FOUR



Page 4-26

Stratigraphy

The following stratigraphy for the site has been adopted from British Coal extraction sections:

Coal Seam Stratigraphy

Rider No.1

Lantern Westphalian B

2ft 9" Middle Coal Measures

Upper 4ft

Lower 4ft

6ft

Northern high wall

Continuous decimetre to metre coal seams and sandstone beds extended east-west along strike

across the high wall with sandstone beds showing thickening towards the east. Evidence was

therefore sought for synsedimentary fault activity from British Coal survey extraction

sections, which were used to construct composite stratigraphic sections for the excavated

sequence across the portion of the site remaining exposed until 8th December 1989. These

sections show that coal seams and intervening sequences have been thickened at different

levels across the site and to variable extents on passing up sequence. The changes may be

ascribed to either synsedimentary or late tectonic effects. Extraction sections were examined

from the site to obtain information on the style of structure encountered during earlier

excavation. The north-south striking extraction sections were spaced at 25m intervals across

the site.

Section 1:

Section 1, along the western side of the site, reveals beds dipping northwards at a moderate

angle eg:

Seam Dip

4ft B 42°N

6ft D 45°N

The lower 4ft seam shows a dramatic thickening at middle levels in the centre of the exposed

site but the seam does not split and the thickening is therefore ascribed to a tectonic origin.

Section 2:

Passing 25m eastwards, lower seams are present on the section. The Lower 6ft D seam and 6ft

D seam show metre scale repetitions. In the northern part of the section, the 6ft D seam is

offset by a thrust with a metre displacement.

CHAPTER FOUR



Page 4-27

In contrast the 6ft D Rider is continuous across the section, however the thickness of the

intervening strata between the 6ft seams increases on passing northwards.

The 6ft B seams shows repetition, whilst the 6ft B Rider is continuous showing a moderate

northerly dip and minor thickness variations.

The 4ft B seam splits at the base of the site and on passing northwards down dip, the

separation between the leaves increases. The Lower 4ft seam shows thickness variations with

moderate metre-scale repetitions occurring in the Upper 4ft seam. The 2ft 9" seam also splits.

The Lantern and Rider No.1 seams are continuous across the section dipping moderately to

the north.

From this section it is apparent that thickness variations are concentrated within levels of the

site occupied by the 6ft and 4ft seams.

Section 3:

Passing eastwards 50m from the western edge of the site, the overall dip of bedding increases

to 52°N. There is evidence for northward directed thrusting (Fig. 4.26a). At the base of the

site there is significant seam repetition continuing to middle and upper levels.

The Lower 4ft B is continuous down to lower bench levels but has been affected by

southward verging thrusts (Fig. 4.26b). The Lower 4ft seam in middle mine levels shows a

distinct thickening which increases down dip to around 1m. The Upper 4ft seam dips 35°N

with repetitions occurring at lower and middle mine levels. The seams above the 4ft seam

show a consistent northerly dip.

Section 4:

Passing 125m eastwards from the western edge of the site there are obvious structures on the

extraction sections. On traversing northwards seam dips reduce progressively: 45°N, 39-

42°N, 31°N to 30-31°N. The most evident repetitions occur in the 6ft D and 6ft C seams. A

major decametre anticline faces northwards in the 6ft B Rider seam. The seam is repeated at

least three times and where it is continuous shows a change in thickness pointing to further

intra-seam shortening. Fig. 4.26c illustrates the structure of the 6ft D Rider seam.

In summary northward verging and downward facing folds are evident as well as southward

verging thrusts and folds as indicated in sections through the Lower 4ft B seam.

CHAPTER FOUR



Page 4-28

Section 5:

225m eastwards from the western edge of the site, the first major structure is observed. A

decimetre to metre wavelength anticline in the 6ft B seam increases in wavelength 25m

eastwards in lower mine levels. On passing eastwards the hinge point apparently moves in

section to upper levels of the mine and from south to north indicating that the fold is plunging

towards the west south-west. A major southward verging syncline occurs to the south of the

anticline. Beds are planar and continuous above these folds. Some thrusting is involved with

the folding, and coal repetition occurs in the northern part of the anticline-syncline fold pair.

Section 6:

In the eastern-most section, 325m eastwards from the edge of the site, the deformation

increases in intensity. Bedding appears overturned and anticlines are truncated by gently

northward dipping beds (Fig. 4.26d). Synclinal folding continues to upper mine levels in the

south of the site. Distinct repetition by southward verging thrusts occurs mainly in the 6ft

Rider and above in the Lower 4ft.

Seams above the 4ft seam dip moderately to the north and appear much thinner than the 6ft

seams in lower mine levels.

CHAPTER FOUR



Page 4-29

4.5.4.1 INFERENCES

The composite stratigraphic sections show that coal seams and intervening strata are

thickened differentially on passing up sequence. This may be due either to the concentration

of thrusting at certain horizons in the stratigraphy resulting from a rheological control, or to

synsedimentary controls on the thickness of the sequence and subsequent controls on the

proportion and distribution of competent sandstone to incompetent mudstone.

Observations by Elliott & Lapido (1981) and Gayer (pers. com. 1990) that synsedimentary

faults were well developed in Park Slip Extension suggest a synsedimentary origin for the

thickness variations (Fig. 4.27) but the evidence above from the extraction sections suggests a

tectonic control.

The main structural history obtained from the site (apart from early synsedimentary

deformation is that early north verging thrusts and folds were tilted towards the north by late

uplift and associated major southward verging thrusting. However this bipartite subdivision of

deformation may be apparent in the site but indeed could represent two different events form

those identified in North Devon.

CHAPTER FOUR



Page 4-30

For example the early fore thrusting event (V1) could be correlated with either or neither of

the North Devon phases. The late back thrusting event (V2) affecting Park Slip Extension and

the remaining south crop of the coalfield is clearly not developed to the same degree in North

Devon unless the Rusey Fault Zone of North Cornwall far to the South is considered.

Therefore it is difficult to correlate the major back thrusting event with Variscan structural

events of North Devon. The former suggestion however can still be made on the basis of the

recognition of two sets of compressional tectonic structures and on the basis of the co-eval

relationship between back thrusting and Vo fore thrusting in Ffos Las (see below). Clearly

there is a problem in correlating structures when considering a probable progressive migrating

deformation.

However the following structural history is postulated (based partly on Chapter 3) (Fig. 4.28):

4.5.5 FFOS LAS

Ffos Las opencast site was initially visited during August 1988. An overview of the main

structure was obtained and some details of the structure of the east face were observed.

However a satisfactory overview of the structure could not be obtained due to the lack of

maturity of the site. Only its complexity was evident.

CHAPTER FOUR



Page 4-31

Detailed research has been carried out in this site by Frodsham (1990) and by Frodsham &

Gayer (1991) who have identified a characteristic style of deformation in the MCM of Ffos

Las named Progressive Easy Slip Thrusting (PEST). It was therefore intended to identify the

regional significance of the structural deformation in Ffos Las in relation to the evolution of

the south crop and the Gower area. This was intended to develop into the detailed analysis of

the late regional back thrusting V2 phase of South Wales. Current research on mesoscale and

microscopic coal structure is being carried out by I. Harris

Later visits revealed a developed picture of the structure. In May and June 1990 the east wall

revealed a nearly complete section of structures (Fig. 4.29a,{b, c & d}) which controlled the

distribution of the Big Vein seam (see below). However a complete section was never seen as

the site does not extend southwards or downwards sufficiently to expose the footwall cut off

the main structure. These have been described in the light of the Progressive Easy Slip

Thrusting and in comparison with other structurally complex sites such as Ffyndaff

Additional (Section 4.5.6).

The stratigraphy of Ffos Las is given in Fig. 4.29e in relation to the proven stratigraphy of

neighbouring collieries Carway and Trimsaran.

CHAPTER FOUR



Page 4-32

General structural results

Early surveys of August 1988 (Fig. 4.30) revealed the scale and the intensity of deformation

affecting the Big Vein seam along a section across the east face. The main northward facing

synform and antiform in the section is complicated by decametre northward directed thrusts

and earlier downward verging and folded thrusts cut by northward directed thrusts. Figs.

4.31a & b show an example of a folded thrust with a hangingwall anticline and footwall

syncline tightened by a later folding event. Further evidence for the dominance of this form of

structure was sought in the later surveys of May 1990.

Large scale decametre northward facing folds dominate the mature east face of the site which

shows a regional southerly dip. An anticline was discovered to have downward facing

structures on its northern limb. Traversing northwards from the southern edge of the high wall

the southern limb of the syncline, to the south of the main anticline, (Fig. 4.29a), is

overturned. Accommodation thrusts pass into the accompanying anticline. Downward verging

thrusts on the northern limb are folded. To the north, bedding within thick coarse sandstone

units representing the Big Rock is flat and is associated with flat thrusts. Thinner units above

the Big Rock show over folding.

CHAPTER FOUR



Page 4-33

Details of the structure observed in the Big E seam on passing further northwards on the

lowest bench level show that intense north facing folds and thrusting in mudrocks are

dominant. These occur below a northward facing fold with distinct shearing occurring below

its hinge within the mudrocks. The main antiform plunges eastwards.

The structure within the Big E seam contains high planar cleat i.e. a centimetre pervasive

fracture fabric which is almost perpendicular to bedding; pyrite banding parallel to bedding;

polished surfaces with minor slickenside lineations developed; larger sets of lineations on

shiny grey-lead grey coal surfaces; and minor kink bands on a decametre scale nearly

perpendicular to bedding (Fig. 4.32).

In the footwall to the main northward directed thrust associated with the folding in the east

face, below the Big Rock, buckling and shear structures occur on a centimetre scale.

Associated quartz vein sets indicate shearing whilst injection structures of shale were

protruded into fractures along the base of the arenaceous beds possibly due to shale over

pressuring as a result of the presence of fluids, (Gayer et al, 1991).

There is good evidence for the occurrence of various types of fluids in the MCM. Work has

presently been completed by S. White on the thermal evolution of the South Wales Coalfield

and further research is being carried out on cleat mineralization by R. Gayer investigating the

nature of the fluids which have traversed through the succession. Present inspection of many

of the sites including Ffos Las reveals the presence of methane gas rising from excavated coal

seams submerged in rain water.

Lower mine levels showed smaller metre scale northward facing structures in the Big E vein

to those seen in the east wall. In the Big C vein fold axes plunge gently towards the west

south-west (Fig. 4.33). Oblique fabrics occur in the coal in relation to the fold axial planes.

Buckling in the coal on a centimetre scale reflects the main fold orientation in the area.

In the north wall at uppermost mine levels the general dip of bedding is towards the north

north-east. Mainly horizontal bedding is undisturbed. However isolated thrust structures with

metre displacements verge towards the north (Fig. 4.34). The size of the folding is smaller but

still northward facing. At the level of the Mole and Mole Rider seams, northward directed

thrusting may be accompanied by decametre scale underthrusting to preserve the gentle dip of

strata higher up in the sequence (Fig. 4.29a).

Following the confirmation of the northward facing structures and possible polyphase or

progressive deformation associated with the thrusting, measurements of the orientation and

CHAPTER FOUR



Page 4-34

CHAPTER FOUR



Page 4-35

complexity of the structure show that repetition occurs at a metre scale within each sub-seam

of the Big Vein seam. At the crest of the main antiform, previously identified in the east wall,

the Big Vein seam is repeated twice on a decimetre scale. In the first repetition, cleat dips

steeply towards the south-east; cleavage within duplexes in mud rocks dips steeply towards

the north north-east. The friability of the coal is variable but is very high in zones suffering a

sinuous pervasive cleavage. The seat earth behaves in a more ductile fashion and shows

folding. In the second repetition, further cleavage duplexes are associated with intersecting

polished surfaces dipping steeply towards the south-west showing the development of

slickenside lineations plunging 22° to 146°.

Fig. 4.35 represents a synopsis of the major structure observed in the eastern face of the site.

The major structure is confirmed to plunge towards the east. A shear zone passing northwards

below the Big Rock is a possible northward extension of the main thrust of the duplex

structure immediately to the south. The Big Rock is folded to the north above the thrust.

Lower in the structure along the Green seam further northward facing decametre structures

occur. At higher levels in the Kings seam and further to the Graigog seam, decimetre fold

structures pass laterally in to the Kings seam. The Graigog shows back thrusting over

northward directed thrusts, which emphasises the discontinuity of some metre fore thrusts.

The relation between the main structure in the eastern high wall and the higher seams is

illustrated in Fig. 4.29a.

CHAPTER FOUR



Page 4-36

The main thrust climbs northwards up sequence to the Kings seam to produce decimetre to

metre folds, possibly folding an earlier flat lying thrust, which contains the Green seam in the

hangingwall. Thrust flats are dominant in the structure, apart from areas of imbricate thrusting

in the Big Vein seam.

4.5.5.1 INFERENCES

The site shows evidence for many cross cutting relationships possibly produced by

progressive thrust development. From the section through the Big Vein seam to the Kings

seam and the overthrust Green seam it can be seen that thrusting may not only be out-of-

sequence but also cuts through into older strata from lower horizons as one phase of in-

sequence thrusting is dissected by a later phase of out-of-sequence thrusting and reoriented by

associated folding. All structures observed were originally northward directed so that a

polyphase or progressive northward translocation is inferred for the Ffos Las sequence. This

could point to an allochthonous origin with major displacements inferred on local

disturbances such as the Trimsaran and Llannon Disturbances. The only distinct evidence of

back thrusting cutting the northward directed structures occurs higher in the sequence at

Graigog seam levels. Underthrusting of the highly repeated sequence below the Graigog seam

may have prevented the same style of shortening from passing to higher stratigraphic levels.

This suggests that the main repetition and translocation of seams occurs in the middle section

of the MCM at decametre to (100m) thicknesses above the Amman Marine Band in this

section of the coalfield.

CHAPTER FOUR



Page 4-37

4.5.5.2 REGIONAL IMPLICATIONS

The structural history of the site may be explained in terms of the two compressional phases

(of North Devon) in which deformation may initially have been concentrated along the

Trimsaran and Llannon Disturbances during a reactivation phase and then greatly accentuated

during later Variscan thrusting to produce the complex thrust relations within the MCM. The

complexity of Variscan thrusting makes the two-fold history less apparent in Ffos Las than in

other sites.

It is also possible that an analogy can be made between Ffos Las and the structure of the south

crop (Jones, 1991) in that a dramatically shortened sequence is observed beneath an

underthrust (discussed in Chapter 5). However the presence of distinct north-westerly faults

and Caledonoid structures between Ffos Las and the eastern Vale area both count against any

direct correlation. This is also implied by the rapidly changing structural style on passing

laterally through the coalfield sites and vertically through the succession. For example, Ffos

Las shows little resemblance to closer structural areas such as the Gower or Porthcawl unless

localities such as Caswell Bay are considered (Fig. 4.36).

In terms of the regional appraisal of the structure of Ffos Las, it may represent a good site to

study the effects of both regional deformation phases affecting the Bristol Channel

Borderlands.

CHAPTER FOUR



Page 4-38

Furthermore, the change in direction of thrusting on passing up sequence in Ffos Las makes

the site a candidate for obtaining the relationship between the major fore thrusting and the

back thrusting of the south crop of the coalfield. Present structural sections from Ffos Las

could point to a coeval relationship between these phases.

4.5.6 FFYNDAFF ADDITIONAL

Introduction

Ffyndaff Additional was visited during November and December 1989 and July 1990. The

site approached the limits and depth of excavation in July 1990 but further interesting

structures were expected to be exposed during August 1990. The site exposed the following

seams and structure:

• 2ft 9"

• 4ft

• 6ft

• Red 2

• 9ft

• Bute

In November 1989, the developing high wall revealed an oblique section through the structure

and exposed mesoscopic thrusts verging towards the north. The 6ft seam and the overlying 4ft

seam were exposed in the wall.

Folds and thrusts deforming the 6ft seam were positioned closely to decametre extensional

faults (Fig. 4.37) which also affected the 4ft seam above. In lower mine levels, towards the

centre of the site the 6ft seam showed 2 leaves with a metre separation, each leaf being up to

2m thick.

Details of the 6ft seam showed that rashings were abundant above the Lower 6ft leaf. The

rashings contained a centimetre pervasive, wavy sinuous cleavage striking east south-east and

dipping moderately towards the south south-west. Convoluted cleavage fabrics were also

developed. Together with these microfabrics, discrete metre to decimetre spaced NNW-SSE

striking slip surfaces occurred in the 6ft seam showing moderately plunging slickenside

lineations directed towards the east north-east. Silty mudrocks above the roof of the Upper 6ft

leaf contained north-east to east north-east facing, metre scale, tight rounded folds (Fig. 4.38).

Further folds were noted to have an anomalous facing direction, with hinge lines plunging 21°

towards 116°.

CHAPTER FOUR



Page 4-39

Overturned folding was associated with early thrusting in the Upper 6ft leaf with no such

equivalent structures in the Lower 6ft leaf. At middle levels, towards the west of the site there

were metre to decametre southward facing folds associated with southward verging back

thrusts and also northward verging fore thrusts.

The planar bedded sequence down to the Red 2 seam, 30m below the 6ft seam, had a regional,

gentle to moderate south-easterly dip. The Red 2 seam was affected by a southward facing

syncline with a moderate dipping northern limb and gently dipping southern limb.

In the southern face at upper mine levels (above the 6ft seam and the 4ft seam) the 2ft 9"

seam had been thickened by northward verging thrusts (Fig. 4.39). An oblique section through

these structures displays a tip fold above one of the thrusts.

Measurements of the orientations of the structures in the site were obtained from a detailed

survey in December 1989. Beds generally dip moderately towards the south. Locally east

north-east striking beds dip moderately towards the north and contain mudrocks with polished

planar surfaces showing pyritic strain fabrics pitching moderately from the west (Fig. 4.40).

Thrusts clearly show a northward directed transportation direction based on the associated

fold geometry and the presence of slickenside lineations which plunge steeply towards the

south.

CHAPTER FOUR



Page 4-40

An east north-easterly high planar cleavage i.e. a cleavage developed at a steep angle to

bedding is also associated with the thrusting. Metre scale folds plunge gently towards the east

south-east and west south-west. They are northward facing moderate to tight rounded folds

related to the thrusts within the Lower 6ft seam.

In summary, the Ffyndaff Additional site contains many northward verging thrusts showing

various age relationships eg early mesoscale thrusts have been reoriented by folding above

late thrusts. Folding is directly associated with the thrusts. Decametre folds have been

observed in the east of the site and metre folds in the west.

CHAPTER FOUR



Page 4-41

East north-east facing folds probably represent lateral structures related to the main northward

verging thrusts and southward facing folds are probably related to back thrusts.

In this site, extensional faults probably associated with the seam splits show an unclear

relation to the compressional structures.

4.5.6.1 OBSERVATIONS OF STRUCTURE EXPOSED DURING RECENT MINE DEVELOPMENTS, JULY

1990

The southern high wall of Ffyndaff Additional was excavated down to the Lower 6ft seam

exposing more of the distinct northward directed thrusts. Structures below a mud/coal lens

(observed previously) were also exposed. Fig. 4.41a shows an outline of a linked northward

directed thrust system interpreted from the structure across the face. Fig. 4.41b shows a view

of the high wall. The Lower 6ft occurs at the base of the section and reappears midway up the

face as the Upper 6ft seam in the hangingwall of a main thrust causing a decametre-scale

displacement. The hangingwall is affected by 3 main decametre-length thrusts, partly

repeating the 6ft seam. Tight folding and minor back thrusting and downward facing thrusting

occurs along with the main thrusting. The Upper 6ft seam shows distinct repetition by thrusts

with long flats and very short metre scale gentle to moderate southward dipping ramps (Fig.

4.42a & b). Most of the deformation is located below the Upper 6ft seam, with tight tip folds

and possible isolated thrusts occurring within and below the 4ft seam respectively. However

thrusting may pass up from the top of the Upper 6ft into the 4ft seam towards the south-

western limit of the section.

CHAPTER FOUR



Page 4-42

A synopsis and photograph of the structure is shown in Fig. 4.43a & b. Thrusting is localised

along coal seams, thin carbonaceous shales and rashings with a well developed cleavage. Bed

parallel movement is a dominant compressional style.

4.5.6.2 INFERENCES

The initial detailed mesoscopic survey of the site revealed anomalous lateral structures as well

as extensional faulting. However, with lower levels exposed the main northward directed

thrusting characteristic of the north crop became evident.

It is clear that thrusting does occur at 9ft seam levels below the 6ft seam and also up to the 4ft

seam. However from the second survey it was clear that the main repetitions have occurred

within the 6ft seam. A minimum kilometre-scale displacement is inferred from the synoptic

sketch of the thrust sequence. This may point to out-of-sequence thrusting, where the

repetition of the 9ft seam occurred before the main repetitions of the 6ft seam in its

hangingwall, followed by the northward propagation of the decametre thrusts observed within

the Upper 6ft seam eg Fig. 4.44.

Another point of importance is that the localisation of deformation to below the 4ft seam may

indicate disharmonic deformation of the Coal Measure multilayer possibly analogous to the

contrast in deformation observed between the Lower and MCM of the site and the gently

dipping Pennant Measures observed to the south. Another possibility is that deformational

intensity is directly related to the height of each seam, in the hangingwall sequence above a

thrust (Fig. 4.43 above).

CHAPTER FOUR



Page 4-43

Fig. 4.45 summarises the synoptic structure of Ffyndaff Additional and shows the

concentration of the thrusting within the 6ft seam below the 4ft seam.

The fold details between the Upper and Lower 6ft seams show that northward directed thrusts

have been folded by later structures and suggest a multiphase northward directed thrusting

event. This event is possibly synchronous with southward directed events within the site and

possibly with the back thrusting along the south crop (in agreement with observations from

Ffos Las). The folded thrusts may subsequently represent an early compressional event.

However such early thrusts are limited in number and scale so that an origin due to

progressive late thrust deformation is favoured in this instance.

4.5.7 GILFACH IAGO

An overview of the site was carried out in December 1989 to examine the details of local

disturbances and the effects of reactivation on the structural style in the coalfield. During July

1990 a survey of the structure was undertaken to investigate consistency of structural trend at

various detailed scales. This survey would show the relative pervasiveness of the Variscan

overprinting event of thrusting in relation to deformation associated with reactivation of the

local disturbances.

The following seams are exposed at Gilfach Iago:

• Rock (Big of Ffos Las, 4ft Group of Park Slip)

• Green

• Ddugaled

• Hwch (2ft of Ffos Las)

• Stanllyd

• Middle Pumpquart

• Upper Trichwart

• Charcoal

• Braslyd Fach

• Saron Brynlloi

In December 1989, at lower levels in the eastern section of the site, major decametre folds, an

anticline, syncline, anticline set, dominated the structure. The cores of the folds were intensely

deformed. At middle mine levels towards the north a decametre anticline was associated with

the inferred extrapolation of the Caerbryn Disturbance.

CHAPTER FOUR



Page 4-44

The associated structure was a tight, rounded anticline asymmetric to the north-west with a

moderate south-eastward dipping axial plane.

In lower site levels towards the east, the core of the main syncline within the fold set

contained tight north-westward facing structures associated with a gently south-eastward

dipping thrust plane. Thrusting was mainly directed towards the north-west along bedding in

northern parts of the structure (Fig. 4.46a & b).

In middle sections along the southern face of the site, a metre to decametre fold showed

vertical changes in wavelength and amplitude. This unlike the previous structure is probably

isolated to higher positions within the sequence. In July 1990, the main syncline was well

exposed further to the west, away from back fill in the east north-east section of the site. It

was represented well within the Trichwart seam. Northward directed thrusting was evident on

a decametre scale, accompanying the north-westward fold asymmetry.

Details of the Trichwart seam showed that intense shearing had occurred along the

coal/rashings contacts and also within the rashings. Tectonised stratigraphic contacts showed

that the Trichwart seam has been repeated twice on a metre scale. Further minor repetitions of

each thrust slice were also found. In conjunction with shearing, deformation within the

Trichwart seam amounts to a repetition of at least seven times (on a decimetre scale).

Accounting for disruption by shearing and cleavage formation the Trichwart seam is up to

four times its original compacted thickness. The seat earth immediately beneath the Trichwart

seam had been sheared intensely and in some decimetre repetition units was consequently

absent. Cleavage duplexes occur on a centimetre-decimetre scale and reflect the duplex

thickening of the coal seam on the north-western limb of the syncline.

CHAPTER FOUR



Page 4-45

4.5.7.1 INFERENCES

Thrusts associated with highly strained cores of folds are interpreted here as accommodation

structures. Bed parallel shear inferred from dip slip lineations in conjunction with the

accommodation structures represent a fold tightening event. This shortening is most apparent

along the incompetent coal seams and associated rashings which contain cleavage duplexes

on various scales. The microstructures may therefore reflect the main folding event associated

with the Caerbryn Disturbance.

Many models can be proposed to explain the structural relationships observed in this site:

1. One continuous north-westerly Variscan compressional phase

2. A bipartite thrust/fold compressional event.

3. A complex thrust/fold/fold tightening event.

4. North-south compressive reactivation of the Caerbryn Disturbance followed by later

compression and associated thrusting.

5. Polyphase deformation or PEST.

Analysis

1. Gilfach Iago could have been affected by north-west - south-east Variscan compression

which produced the main north-east/south-west trending folds. In later stages of this

compression, the accommodation structures could have developed together with coal seam

thrusting, cleavage formation and bed parallel shearing.

2. It is unclear from local structure whether thrusting within the Trichwart seam pre- or post-

dates the folding (Fig. 4.46c). The asymmetry and facing of duplexes within this seam is one

line of evidence which can be used to infer a late tightening event during one main

compressional phase, as in (1) above. However the large amount of repetition of the seam and

its lateral continuity does point to the thrusting continuing across the fold hinge and thus

being earlier than the folding event, with the present orientation of the microstructures and

thrusts being produced during a later fold tightening event. This evolution therefore suggests

an early event of main thrusting was succeeded by the folding event.

CHAPTER FOUR



Page 4-46

CHAPTER FOUR



Page 4-47

3. Combining models (1) and (2) would result in a late fold tightening and shearing event of

deformation following early thrusting and main folding events. Model 3 may account for the

smaller scale repetitions of the Trichwart seam slices and their structural orientation in

relation to the folding.

The exact age and origin of the main folding event is questioned here in relation to the

regional structure and in the light of further observations of major structures in Gilfach Iago.

These have yielded new structural data on the age relationships within the site and a further

model for the origin of the folds:

4. In agreement with Cole et al (1991) folding in Gilfach Iago is attributed to the reactivation

of the Caerbryn Disturbance. The folding and mesoscale structures on the limbs of the major

anticline close to the disturbance (Fig. 4.47) reflect the compartmentalisation of a regional

north-south principal compressive stress into a NW/SE compressive stress and NE/SW shear

stress (in relation to the disturbance) (Fig. 4.48). Considering (2) above, this suggests that

thrusting also occurred prior to the reactivation phase. An excellent example of a major thrust

folded by an anticline related to the disturbance is evidence for this inference (Fig. 4.49).

Further observations show that Gilfach Iago contains decametre scale thrusts and possible

duplex (Fig. 4.50) which appear to cut through bedding regardless of dip which suggests that

these thrusts post-date the folding related to reactivation of the disturbance and may therefore

represent late Variscan structures.

CHAPTER FOUR



Page 4-48

I.e. Gilfach Iago contains structures reflecting a composite deformation history. One further

model (5) can be suggested in that this composite deformation history could have been

produced by PEST eg PEST could account for thrusts developing at high levels being

deformed by tip folds at lower levels.

4.5.8 EAST PIT EXTENSION

Introduction to Variscan structures post-dating Variscan thrusts

East Pit Extension was examined to show the effects of Late Variscan extensional faulting on

older structures and on the detailed coal structure within the site. East Pit Extension is ideal

for this survey (together with Nant Helen, Derlwyn and Benward's Field) because the

structure of the site is dominated by an eastward facing monocline which is positioned above

the buried eastward dipping cross fault - the Cwm Teg Fault (proved in deep mines). This

high level monocline is observed in the east-west striking high wall of the site to be related to

westward dipping extensional faults conjugate to the Cwm Teg Fault below. In order to

examine the style of faulting and its relation to the earlier thrusts this faulting was examined

in the high wall and traced laterally across the site. East Pit Extension exposed the following

coal sequence:

• Black Band

• Soap

• Penny Pieces

• Stwrin

• (Upper White) White 4ft

• Upper Black

• Pencraig

• Harnlo

• Big

In December 1989, close inspection of the east-west trending high wall showed that it formed

an oblique section to the east north-east facing monocline. Investigation of the faulting related

to the monocline showed that there were minor extensional faults, fracture cleavage and

jointing which dipped steeply towards the west south-west (Fig. 4.51). Larger faults had a

distinct ramp-flat geometry and passed westwards into parallel fracture cleavage. All these

structures had a very similar trend to the monocline and appeared unfolded which suggests

that they were also late structures.

CHAPTER FOUR



Page 4-49

However in middle to upper mine levels in the northern benched face (opposite the high wall)

decimetre micro folds asymmetric to the north-west were associated with bed parallel

decametre-length thrusting found at the base of a competent sandstone. The asymmetry of the

micro folds in the footwall to the thrust and the general younging direction inferred from the

footwall sequence suggest the thrust were clearly cutting down sequence. This is good

evidence for pre-monocline thrusting in the site.

In May 1990, the three dimensional geometry of the monocline was examined. It was traced

northwards across the site where its effects on coal structure were examined. In the southern

section of the site, the Big Vein seam was folded by the monocline. Tracing this particular

horizon across the site showed that the wavelength of the monocline increased on passing

northwards. This indicates that the structure plunged towards the south.

The late faults related to the monocline had obvious effects on coal structure. In upper mine

levels of the southern section of the site there was clear evidence for extensional faulting.

Low angle faults and shear zones had listric geometries and showed mainly normal drag. The

faults clearly displaced the Middle Soap seam but displayed an unclear basal detachment

beneath this seam. In one instance a listric fault had a base coinciding with a south-westward

directed flat of a decametre ramp/flat thrust (Fig. 4.52) 15m beneath the Middle Soap seam.

The scale of listric faulting varied from decametre to metre scale. There was no evidence for

synsedimentary growth along these faults.

CHAPTER FOUR



Page 4-50

Further evidence was sought from seams to the north of the site for similar detailed effects of

the late structure on coal geology. Details of the Middle Soap seam showed that ductile

deformation had occurred in the coal possibly in response to the monoclinal folding. Similarly

at Upper White seam levels northward directed cleavage duplexes were developed in muddy

coal bands, whilst more competent horizons showed metre folds with fold axes plunging

moderately due south. (These may however represent early lateral structures). In either case

these minor folds followed the trend of the monocline.

Other structures within the Upper White seam also represented an earlier compressional

deformation causing major thrust repetition of seams within the site. Conclusive evidence that

the cross faulting post-dates the early compressional structures also occurred within the Upper

White seam in which thrust related folds were cross cut by late extensional faults.

I.e. Observations of the general structure and detailed coal geology suggest that the

monoclinal folding and cross faulting was a late post-compressional event.

Recent exposure at deep levels of the eastern part of the site, revealed good evidence for NNE

transporting thrusts, eg Fig. 4.53a & Fig. 4.53b, between the Big seam and the Harnlo seam.

Hangingwall anticlines above the thrusts plunged at about 10° towards the ESE which

compares well to the gentle easterly dip due to the major monocline. This is taken as further

evidence suggesting that the monoclinal folding post-dated the thrusting i.e. the amount of

plunge on the folds is directly related to the dip due to monoclinal folding.

CHAPTER FOUR



Page 4-51

In terms of the level of thrusting, seams equivalent to the upper part of the sequence of Ffos

Las are generally undeformed in East Pit Extension from which it could be concluded that the

level of thrusting in this section of the coalfield is concentrated to Harnlo levels, below

horizons in which thrusting is dominant further west.

The presence of synsedimentary tectonic extension was also investigated. The relative

importance of extension during the evolution of the South Wales Coalfield was examined in

East Pit Extension with no result. Other sites containing cross faults were also examined.

4.5.9 NANT HELEN AND THE CRIBARTH DISTURBANCE

Nant Helen opencast site lies close to the Swansea Valley Disturbance. The Swansea Valley

Disturbance extends north-eastwards through the north crop and is found to fold the

Carboniferous Limestone. The following map (Fig. 4.54a, b & c) illustrates the geometry of

the south-western section of the anticlinal fold representing the expression of the Swansea

Valley - Cribarth Disturbance in the limestone:

The anticline trends NE-SW and is associated with a complex of tension gashes and

slickenside lineations (Fig. 4.55b & c) which may indicate the compartmentalisation of a

Variscan N-S maximum compressive stress in to a NW directed normal stress and a NE

directed shear stress (Weaver, 1975) (Fig. 4.55a).

CHAPTER FOUR



Page 4-52

It is likely that the structure of Nant Helen is related to the Cribarth Disturbance. In particular

the Pwllau Bach Fault in the site has a long extensional movement history part of which may

be equivalent to the formation of N-S trending extensional tension gashes which are prolific

in the Cribarth area.

CHAPTER FOUR



Page 4-53

The following section describes the structure of the Nant Helen site.

Nant Helen contains early synsedimentary structures (Hartley & Gillespie, 1990), late thrusts

(Gillespie, 1991) and is transected by a major cross fault, the Pwllau Bach Fault (mentioned

above). This points to Nant Helen being an ideal site for the investigation of major cross

cutting relationships. The site exposes the following coal seams:

• Rider

• Soap

• Penny pieces (2ft 9")

• Stwrin (Driver)

• White 4ft (Upper 4ft)

• Black (Lower 4ft)

• Cornish (Red Vein)

• Harnlo (6ft)

• 9ft

• Brass (Bute)

(Derlwyn Site):

• Lower Brass (Yard)

• Bluers (7ft)

• Rhyd (5ft)

• Grey

• New (Gellideg)

The site was investigated for evidence for synsedimentary deformation. Initial inspection of

the site in upper levels revealed granular conglomeratic to medium grained quartz wackes

with a poorly sorted immature sedimentary texture. The sandstones contained decametre to

metre scale cross beds with decimetre bedded cosets. An approximate north-westerly

palaeocurrent direction was obtained from the cross beds. These sandstones occupied the

same stratigraphic horizon (Stwrin Sandstone) as the fault-controlled channel sandstones

described by Hartley & Gillespie (1990). However no further evidence for early

synsedimentary deformation was observed.

Only north-south metre spaced joints with a N/S cross fault trend occurred in upper mine

levels.

CHAPTER FOUR



Page 4-54

Evidence was therefore sought for Late Variscan structures and cross faulting in lower mine

levels. The Pwllau Bach Fault was found to be excellently exposed during later surveys of

March and July, 1991 in middle levels.

The Pwllau Bach Fault was exposed in the east of the site where it consisted of a 7m wide

fault zone striking NNW-SSE and dipping moderately towards the west (Fig. 4.56). A major

fault plane within the zone displayed slickenside lineations plunging moderately towards the

west (268°, 281°). From site extraction sections the Pwllau Bach Fault has a westerly

downthrow of 80m. Recent observations of the fault plane revealed dextral strike-slip

slickenside lineations on planes oblique to the Pwllau Bach Fault. The strike-slip lineations

pre-date the larger dip slip sets.

In the hangingwall of the fault there were eastward verging thrusts deforming the Harnlo and

Cornish seams but not the Soap Rider seam. Displacements of about 6m and 27m were

measured along these thrusts. The thrusts striked 355° to 340° and dipped gently (15°) to

moderately (45°) towards the west. Bedding dipped about 10° towards the west so that the

thrusts appeared to be low-angle ramps. The ramps climbed eastwards to Black seam levels

where flat thrusting and cleavage duplexing within this seam accommodated for the

shortening. This Black seam flat may have linked to another ramp in the hangingwall of the

Pwllau Bach Fault to the east (Fig. 4.57).

CHAPTER FOUR



Page 4-55

4.5.9.1 INFERENCES ON STRUCTURAL HISTORY

Nant Helen reveals good structural evidence for the various coalfield structural phases. Local

synsedimentary extension clearly predates the thrusting and cross faulting. However the

relation between the latter two is unclear. Furthermore as stated previously in this chapter

Hartley (pers. comm., 1990) has demonstrated that the Pwllau Bach Fault had a long

movement history which began with synsedimentary extension. This contradicts the model

that cross faulting was solely a post-thrusting event.

i.e. though on first inspection Nant Helen appears to contain a cross section of the structural

population produced during phases 1 to 4 i.e. early extension, reactivation, late thrusting and

late extension in reality the structures may represent various structural histories, eg:

Case A

(1) Synsedimentary extension along growth faults and active east-west extension along

the Pwllau Bach Fault.

(2) north-south compression causing further extension along the Pwllau Bach Fault.

(3) further north-south compression causing strike-slip faulting and east-directed thrusts.

(4) cross faulting i.e. late extension along the Pwllau Bach Fault.

CHAPTER FOUR



Page 4-56

Case B

(1) Synsedimentary faulting.

(2) Main compression forming east-verging thrusts which may not be ascribed to a general

Variscan compression.

(3) Late cross faulting.

Case C (from site evidence)

(1) Synsedimentary Extension.

(2) Strike-slip

(3) Eastward directed thrusting and extensional cross faulting.

In conclusion Nant Helen reveals eastward directed thrusting (very similar to that observed

along the east crop and in Garnant) which is difficult to correlate with a two-fold

compressional event.

Secondly it is likely that the Pwllau Bach Fault was reactivated prior to the late extension.

The relation between the OCCS and the local Swansea Valley Disturbance (SVD) is

unsolved. However if the Pwllau Bach Fault of Nant Helen had a long movement history

similar to the SVD then they should have been kinematically linked. Weaver (1975)

postulated sinistral strike-slip movement on the SVD due to north-south compression. Early

extension along the Pwllau Bach Fault may have been related to a phase of east-west tension

due to the north-south compression. Later thrusting in the site may have been reoriented by

further strike-slip movement along the SVD or may represent a compartmentalisation of NE-

SW sinistral strike-slip into E-W compression and N-S dextral strike-slip. This may explain

the occurrence of the dextral strike-slip slickensides. The consequence of this latter

interpretation is that the SVD and Pwllau Bach Fault bounded a wedge which may have been

transported to the NNE along oblique and lateral ramps, i.e. SVD and the Pwllau Bach Fault

respectively. Argument against this model can be well founded on the observation of little

disturbance-related deformation in a neighbouring site (Derlwyn). This site only contains

evidence for layer parallel shearing along coal seams and normal cross-faulting (Fig. 4.58). If

reactivation and fault linkage was a dominant deformation style then thrusting would have

been expected even below Brass seam levels. In contrast, thrusting appears to be dominant

much higher in the sequence at Harnlo and Cornish levels. This suggests that the only likely

link between the movement history of the SVD and the Pwllau Bach Fault was the late E-W

extension, well defined in the Cribarth area by the prolific N-S trending tension gashes.

CHAPTER FOUR



Page 4-57

4.5.10 BENWARD'S FIELD

Benward's Field is a small privately operated site near the east crop which exposes a section

of stratigraphy close to the Amman Marine band (Fig. 4.59). The site exposes the following

coal seams:

• Black (worked out)

• Bydylog (equivalent to Upper Bute)

� Amman Marine Band

• Meadow (Yard)

• 5ft (Old Gellideg)

At the base of the site there is the old Gellideg adit in levels equivalent to the 5ft seam.

CHAPTER FOUR



Page 4-58

Structurally the site reveals evidence for 1. lag faulting; 2. thrusting; 3. cross faulting.

Lag Faulting and Thrusting:

Folds and thrusts directed towards the north-west with slickenside lineations plunging towards

the south-west (Fig. 4.60) either point to an anomalous structural trend or possibly

compressional structures developed later upon an early south-easterly directed

synsedimentary extensional fault or late lag fault. The complexity of the structure favours a

composite history.

Cross Faulting:

A north-westerly cross fault transects the southern wall of the site. The fault consists of a

narrow zone of shearing in which bedding blocks have been rotated in a normal sense (Fig.

4.61). Models for the movement associated with the fault are given in Fig. 4.62.

Small scale evidence for normal faulting occurs in the Meadow seam. Further investigation

during April 1991 revealed a series of extensional faults. A particular fault in this set has a

downthrow which increases on passing south-eastwards; others have a low angle of dip (Fig.

4.63) and a downthrow of about 10m.

CHAPTER FOUR



Page 4-59

Coal tends to be sheared downwards along the fault plane as an incompetent lubricating

horizon. Later investigations during May 1991 showed that normal faulting cross cuts the

Bydylog seam with a trend of 294°, a dip of 64°N and a normal downthrow of 2m.

Benward's Field therefore reveals good evidence for limited thrusting in Lower Coal Measure

levels and for late cross faulting.

4.5.11 GARNANT

Garnant contains a variety of structures within two sections of the site, areas A & B.

Coal Stratigraphy

• Stwrin (Driver)

• Upper White

• Upper Black

• Upper Pencraig (above 3 equivalent to 4ft)

• Harnlo (6ft)

• Big

• Lower Black

• Peacock (above 3 equivalent to 9ft)

• Trigloin (Bute)

CHAPTER FOUR



Page 4-60

The structure in area A contains the Lower Black seam and is dominated by mesoscale

thrusting. eg The original thickness of the Lower Black seam is about 60cm. Thrust repetition

of this seam has increased its mineable thickness to 4m. Sigmoidal cleavage is associated with

the thrusting. Cleavage duplexes in the rashings and early thrusts are deformed by early

folding and are cut by late layer parallel thrusting. The Lower Black seam also contains

numerous slip surfaces.

On a larger scale, upper levels of the north-eastern wall of the site contain the Upper Black

seam deformed by north-eastward verging folds and thrusts which may cut down sequence. In

the south, the high wall of the site reveals further decametre north-eastward facing anticlinal

folding whilst in the west the Upper Black seam is succeeded by gently dipping planar bedded

siltstones.

Area B contains the high quality Peacock seam and seams from the Lower Black to the

Pencraig. In the south, east north-eastward directed decametre scale thrusts occur below the

Big Vein seam and extend up to the Pencraig seam which is folded by a structure verging

towards the south most probably concealing a back thrust at depth. A regional southerly dip

dominates this area.

Re-examination of area A during April 1991 revealed in the south-eastern section of the area a

possible tip fold above a reactivated extensional fault (Fig. 4.64). Sandstones are observed to

thicken across the fold towards the north. Another possibility is that the folding above a thrust

took place syndepositionally to affect the thickness of the sandstone. Further inspection may

point to thickness variations across thrust faults being due to the deformation of a channel-

shaped sandstone.

CHAPTER FOUR



Page 4-61

Structurally fold hinges trend 330° with fold asymmetry being predominantly towards north-

east. Associated thrust transport is towards 060°. The Harnlo seam outlines this major system

in the high wall (Fig. 4.65a). Fig. 4.65b represents a high wall section of thrust repeated

sandstones.

At lower bench levels towards the middle of the site, the Lower Black seam here is heavily

sheared and contains minor folds plunging 18° towards 303°. A top to the north-east shear is

observed in this seam. (91cm of the upper section of the Lower Black seam is sheared whilst

134cm of the seam is unsheared) (Fig. 4.65c).

Model: It is possible that the layer parallel thrusting observed in the Lower Black seam is

cross-cut by the ramp thrusts observed in the Harnlo seam, whilst flats above the Harnlo seam

remain unaffected.

4.5.11.1 INFERENCES

Garnant reveals good evidence for further north-easterly verging thrusts (similar to Nant

Helen). It is also possible as in the above model that polyphase thrusting and folding affected

the site in a primitive PEST style. Furthermore after observations of sandstone thickness

variations across structures it is likely that some of the deformation was early; eg reactivation

related folding and/or extension followed by late phase north-easterly directed thrusting.

CHAPTER FOUR



Page 4-62

i.e. though Garnant contains anomalously trending thrusts the structural style is similar to

other sites such as Ffyndaff Additional.

4.5.12 PEN BRYN OER

The general character of Pen Bryn Oer is one of far less deformation than sites in the west.

The stratigraphy is also condensed:

Stratigraphy

• Elled Rider

• Elled (2ft 9")

• Big (equivalent to 6ft & 4ft)

The 4ft seam, forming the upper part of the Big seam, consists of two leaves: an Upper 4ft

leaf separated from a Lower 4ft leaf by a mudrock parting varying in thickness from 10 to

40cm. The 4ft leaves thicken towards the east of the site and together with the 6ft seam form a

Big seam about 7m thick.

Both the 4ft and the 6ft leaves of the Big seam show bed parallel shear with slickenside

lineations plunging 16° towards 135°.

Investigations during April 1991 concentrated on channel-like lenses above the Big seam

showing discordant bases (Fig. 4.66). These channels trend approximately north-south similar

to the cross faulting and are filled with fine grained homogeneous sandy siltstone.

At upper mine levels the Elled and three Elled Rider leaves dip gently towards the south.

Cross faulting in the site trends 353° and dips 90°. An unusual sense of movement is observed

along the strike of the fault. Passing southwards towards the high wall of the site the fault

downthrows 40cm to west. On passing northwards, the fault downthrows 1m to east (Fig.

4.67). This cross fault is described here as a scissors fault.

4.5.12.1 INFERENCES

The dominant style of deformation in this site, probably associated with late compression, is

bed parallel shear within the Big seam leaves.

CHAPTER FOUR



Page 4-63

An earlier phase of synsedimentary tectonic movement can be inferred on two grounds: (1)

the condensed Coal Measure sequence of the Big seam in comparison with the sequence of eg

Ffyndaff (6ft & 4ft) i.e. restricted subsidence; and (2) the trend of channels probably

associated with early cross faulting.

The metre scale torsional scissors movement on the cross fault in Pen Bryn Oer represents the

latest Variscan movement in the site and is probably of the late extensional phase.

4.6 DISCUSSION ON THE GEOLOGICAL HISTORY OBSERVED IN THE OCCS

The structures observed in the OCCS have been described and related to particular phases of

Variscan deformation. As indicated by the numerous cross cutting relationships observed, the

phases of deformation are not distinct within the coalfield. In fact the deformation, eg as

observed in Ffos Las and Gilfach Iago, points to many events of Variscan compression.

Furthermore various fault movements eg in Nant Helen, point to a long structurally

diachronous and variable history. Other sites such as Benward's Field and Pen Bryn Oer, in

the east, record only local evidence for structural deformation such as layer parallel shearing

and do not contain any evidence for discrete cross cutting relationships.

CHAPTER FOUR



Page 4-64

Thus it can be argued strongly that the deformation in the coalfield was variable, progressive

and involved basement lineaments at various times during the Variscan compression rather

than during a discrete reactivation phase.

Conversely it can be argued for the occurrence of two-fold deformation if the likelihood of

reactivation taking place in the foreland of the main orogen is taken in to account.

Consider the following: fault movement due to reactivation (Vr) would be expected to occur

prior to the shift of the orogenic front in to an area and hence prior to the accompanied

formation of new faults (Vo) for the following reason: the criteria below (outlined in Chapter

1) have been met in the South Wales Coalfield:

1. favourable stress orientations

2. favourable fault dips in relation to (1) above

3. lowered cohesive strength of the faulted rock

4. favourable coefficient of friction

5. presence of fluids

eg (E/W) Devonoid normal faults and (NE/SW) Caledonoid Disturbances have been

reactivated in a (N/S) Variscan compressive regime. Fluids were present in the coalfield

which would have assisted with (4) above.

i.e. since there is good evidence that reactivation has taken place and that the criteria for

reactivation were established early in coalfield history then its likely occurrence - prior to the

onset of main compression - should also be expected.

A modification of this model can be proposed in relation to faulting observed in the coalfield:

that reactivation is likely to occur firstly in a foreland area but is also likely to continue as the

orogenic front migrates forelandwards. An evolution dominated by reactivation grading into

orogenic overprinting and structural complexity and diastrophism would thus be expected.

4.7 DISCUSSION ON REGIONAL STRUCTURE IN RELATION TO THE

STRUCTURES OBSERVED WITH THE OCCS

Observations of the stratigraphic horizons in which thrusting has taken place gives an

indication of the geographic variation in the level at which main-phase Variscan thrusting

took place. Sites such as Park Slip Extension and deep mine workings on the south crop

(Jones, 1989) point to back thrusting occurring from Lower Coal Measure levels to the 6ft

CHAPTER FOUR



Page 4-65

seam in the MCM and up to Pennant Measures. However, on the north crop, back thrusting

occurs in Ffos Las at Graigog levels of the MCM. In other sites on the north crop, such as

Ffyndaff and Nant Helen, fore thrusting generally occurs from 9ft to 2ft 9" levels (Harnlo to

Stwrin levels) and not below Lower Brass levels of eg Derlwyn OCCS near Nant Helen.

However immediately south of the coalfield thrusting is imaged on seismic sections to occur

within Lower Palaeozoic strata (Chapter 5). This points to a rise in the stratigraphic level of

thrusting on passing northwards through the coalfield and also on passing westwards. This

indicates that the highest stratigraphic level of thrusting in the MCM would be expected to be

found in the north western section of the coalfield (in which coal rank is highest).

The change in stratigraphic level of thrusting may have been brought about by lateral

structures such as the basement continuation (root structures) of the north-westerly trending

cross faults. These may separate areas of high level thrusting from areas of low level

thrusting.

Jones (1989) stated that the level of fore thrusting is also stratigraphically located beneath the

Pennant Measures and inferred major underthrusting beneath the south crop. This is possibly

equivalent to back thrusting observed in upper parts of the Ffos Las section. i.e. It is possible

that the structure immediately south of the coalfield in the Vale of Glamorgan is a deep level

structural equivalent to the structure of the western part of the coalfield (see section 4.8

below). An alternative interpretation of the Variscan structure beneath the Vale of Glamorgan

is given in Chapter 5.

The overall change in stratigraphic level of thrusting could have been caused by a raising

effect of elevated basement (Hillier, 1992) on Variscan thrusting in the western part of the

coalfield. The deeper basement to the east would allow thrusts to develop freely at lower

stratigraphic levels. Two possibilities exist to explain the rise in level of thrusting:

1. thrusting ramped upwards along a steep buried fault plane bounding the southern flank of

the basement topographic high (Fig. 4.68a);

2. thrusting used the weakest horizon draped over a smooth incline up to the topographic high.

In this case thrusting would be in a flat style in relation to bedding but would revert to a ramp

and climb up sequence as the dip of strata changed to horizontal above the basement

topographic high (Fig. 4.68b).

i.e. Basement topography could account for the level of sole thrusting throughout the

coalfield. Furthermore the shortening effects of underthrusting (Jones, 1991) beneath the

CHAPTER FOUR



Page 4-66

south crop and to the west in Ffos Las could account for roof thrusting in upper levels of the

MCM and also up to the Pennant Measures. This feature may also have been due to basement

topography, in this case buttressing against a south-dipping basement incline beneath the

south crop.

4.8 COALFIELD STRUCTURAL ENVIRONMENTS

It has been demonstrated in section 4.5.4-12 that the coalfield contains thrusts, extensional

faults and some strike-slip deformation. The OCCS investigations show that these structural

styles co-exist and show variable age relationships. However it is interesting to note that even

though the South Wales Coalfield shows a complex structural framework it can be subdivided

into a number of areas or structural environments in which a predominant structural style

exists. The following map (Fig. 4.69) illustrates this point.

Area A

The eastern section of the coalfield is dominated by predominantly westward downthrowing,

NW-trending, extensional, cross faults, NE-directed thrusts and NE-trending folds.

Area B

The area along the south crop (south of the Pontypridd Anticline) is dominated by E-W

trending strike thrusts which are generally S-directed back thrusts. Cross faulting is limited in

this area.

CHAPTER FOUR



Page 4-67

Area C

This area is localised around the anomalous deformation associated with the E-W trending

extensional S-downthrowing Moel Gulau Fault which separates Area B from Area A. Folding

in this area is E-W trending.

Area D

This area lies between the Neath and Swansea Valley Disturbance and thus represents a

natural segment of the coalfield bounded by Caledonoid structures. It contains N-S trending

cross faults which occasionally extend southwards in to Area C and marks the change from

westward downthrowing cross faulting to generally eastward downthrowing cross faulting.

This area also contains anomalous fold trends along the north crop which includes NE & E-

directed thrusting and NE trending folds.

Area E

This area forms the western part of the coalfield and contains NNW-trending cross faults

which mainly downthrow to the east and folds which trend E-W to ENE-WSW. The area

consists of two distinct sub-areas E1 and E2: E1 is the southern section and is structurally

similar to Area B whilst E2 is the northern section and it contains structures similar to the

north crop of Area D. E1 and E2 may represent a similar subdivision of the coalfield as in

Areas A & B.

E1 contains fore thrusts trending approximately E-W whilst E2 contains disturbances trending

ENE-WSW similar to the neighbouring Carreg Cennen Disturbance. Thus it is likely that E1

contains mainly pristine Variscan structures whilst E2 contains reactivated disturbances.

CHAPTER FOUR



Page 4-68

Area F

This area consists of the Pembrokeshire coalfield. Deformation along the Littlehaven to

Broadhaven coastal section (Hancock et al, 1982) was examined during September 1988 and

revealed a series of N-directed major thrusts and related folds (Figs. 4.70) which are similar to

the more complex of folds observed in the western part of the main coalfield. (Structural

deformation along this section is attributed to high level thrusting within the Coal Measures

and is thus correlated with Sub-area E1).

4.9 FORELAND BASIN DEVELOPMENT RELATED TO FAULT MOVEMENT IN

THE BRISTOL CHANNEL (THE BCT & GMT)

The above subdivision of the South Wales Coalfield into structural environments can be

extended southwards into the Bristol Channel, beneath which a compressional Late

Palaeozoic structural environment is expected (Chapter 6): a structural environment

dominated by ESE-trending thrusts formed by reactivation of Devonian extensional faults in a

similar structural style to SW Dyfed.

CHAPTER FOUR



Page 4-69

It is likely that the South Wales foreland basin to the north was affected by faulting to the

south to a degree similar to that caused by reactivated faults actually beneath the basin. It is

postulated here that the BCT and GMT beneath the Bristol Channel formed the northern

extent of a composite landmass extending southwards into Cornwall which had a local

loading affect on the southern margin of the basin (Chapter 2) even though it would be

expected that the effect of flexural rigidity of the foreland would result in widespread loading.

In support of this hypothesis the deepest part of the foreland basin lies immediately to the

north of the BCT and GMT. i.e. the GMT may have caused a large local uplift to produce a

topographic high significant enough to cause a preferential deepening of the basin north of the

Gower. Since the Millstone Grit also thickens towards the south crop, fault movement along

the BCT and GMT probably occurred as early as Namurian times.

4.10 TIMING OF VARISCAN MOVEMENT ALONG THE BCT AND GMT

Definite evidence for the movement of the BCT and GMT comes from the southerly

derivation of the Westphalian C Pennant Measures (Gayer & Jones, 1989).

Petrographic lithic clast studies of the Pennant Sandstone would give an indication of the

lithic source types to the south and an indication of the age of uplift. Previous petrographic

studies have concentrated on diagenetic aspects of the Pennant Sandtsone eg (Khalaf, 1980).

A source area investigation however could reveal good evidence for the extent of uplift in the

hinterland during the Westphalian C using a knowledge of the petrographic characteristics of

the formations of North Devon.

Another line of investigation which could be followed, to date the Variscan movement along

the BCT and GMT, would be to examine the unconformable relationships within the Bristol

Coalfield to the east.

Examination of BGS maps of the area reveals an overstep of the Pennant Measures onto

Lower Coal Measures and older strata. This indicates an episode of movement between the

Westphalian B and lower part of the Westphalian C. However the thickness of the Millstone

Grit in the Bristol area is much thinner than along the south crop which could point to an

earlier phase of marginal uplift in a similar style to the east crop. i.e. in light of the composite

movement histories observed in the South Wales Coalfield it is likely that faults beneath the

Bristol Channel also moved prior to the onset of the main Vo Variscan compression.

CHAPTER FOUR



Page 4-70

4.11 CONCLUSION

The examination of the opencast coal sites reveals structural styles and structural age

relationships clearly showing that Late Palaeozoic tectonism involved a number of movement

phases which involved reactivation and affected the regional stratigraphy of the South Wales

area.

Examination of disturbances within the coalfield points to an early Variscan reactivation Vr

phase being overprinted by a main compressional Vo phase. Overprinting in some instances

obliterates the bi-partite structural record.

4.12 APPLIED COAL GEOLOGICAL RESULTS

The investigation of the opencast sites has revealed a number of characteristics of the South

Wales coalfield which can been exploited for the benefit of coal mining in the area. Firstly

observation of the style of faulting in the opencast sites allows the formation of a template

with which structural data from the whole coalfield can be interpreted. Secondly the

subdivision of the coalfield in to structural environments could assist in the drawing of

realistic cross sections through a prospective area, i.e. sections agreeable with the local

geology. Thirdly a knowledge and the prediction of the stratigraphic level of thrusting would

assist in the mining industry since mines positioned above thrusted horizons would contain

the greatest repetitions of coal seams. This would be a hindrance to underground mining but

would be beneficial to opencast mining.

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REFERENCES

Allen, J.R.L., 1962. Lower Old Red Sandstone of the Southern British

Isles: a facies resembling the Alpine Molasse. Nature,

193, pp. 1148-1150.

Allen, J.R.L., 1964. Pre-Pickwell Down age of the Plateau Beds

(Upper Devonian) in South Wales. Nature, London, 204,

pp.364-366.

Allen, J.R.L., 1965. Fining-upward cycles in alluvial successions.

Geological Journal, 4, pp. 229-246.

Allen, J.R.L., 1965. Upper Old Red Sandstone (Farlovian)

palaeogeography in South Wales and the Welsh Borderland.

Journal of Sedimentary Petrology, 35, pp.167-195.

Allen, J.R.L., Bassett, M.G., Hancock, P.L., Walmsley, V.G. &

Williams, B.P.J., 1976. Stratigraphy and structure of the

Winsle Inlier, south-west Dyfed, Wales. Proceedings of

the Geologists' Association, 87(2), pp. 221-229.

Allen, J.R.L. & Tarlo, L.B., 1963. The Downtonian and Dittonian

facies of the Welsh Borderland. Geological Magazine, 100,

pp. 129-155.

Allen, J.R.L. & Williams, B.P.J., 1978. The sequence of the earlier

Lower Old Red Sandstone (Siluro-Devonian), north of

Milford Haven, south-west Dyfed (Wales). Geological

Journal, 13, pp.113-136.

Anderson, J.G.C., 1971. The Cardiff District. Geologists'

Association Guides No.16. Edited by J.G. Capewell.

Anderton, R., Bridges, P.H., Leeder, M.R. & Sellwood, B.W., 1979. A

Dynamic Stratigraphy of the British Isles. A study in

crustal evolution. George Allen & Unwin, London.

Archer, A.A., 1968. The Upper Carboniferous and later formations of

the Gwendraeth Valley and adjoining areas (special

memoir). Memoir of the Geological Survey of Great

Britain.

Baker, J.W., 1971. The Proterozoic history of southern Britain.

Proceedings of the Geologists' Association, 82,

pp. 249-266.

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.

Ball, H.W. & Dineley, D.L., 1961. The Old Red Sandstone of Brown

Clee Hill and the adjacent area. I. Stratigraphy.

Bulletin of the British Museum of Natural History. A5,

pp. 177-242.

Bayerly, M. & Brooks, M., 1980. A seismic study of deep structure in

South Wales using quarry blasts. Geophysical Journal of

The Royal Astronomical Society, 60, pp. 1-19.

Blake, J.F., 1884. On the volcanic group of St. David's. Quarterly

Journal of the Geological Society of London, Vol. 40,

pp. 294-311.

Blundell, C.R.K., 1952. The succession and structure of the

North-eastern area of the South Wales Coalfield.

Quarterly Journal of the Geological Society of London,

107, pp. 307-333.

Cole, J.E., Miliorizos, M., Frodsham, K., Gayer, R.A., Gillespie,

P.A., Hartley, A.J. & White, S.C., 1991. Variscan

structures in the opencast coal sites of the South Wales

CHAPTER FOUR



Page 4-72

Coalfield. Proceedings of the Ussher Society, 7,

pp. 375-379.

Cope, J.C.W., 1979. The early history of the southern margin of the

Twyi anticline, in the Carmarthen area, South Wales. In:

Harris, A.L., Holland, C.H. & Leake, B.E. (Eds.). The

Caledonides of the British Isles - reviewed. Geological

Society of London, Special Publication, 8, pp. 527-532.

Crimes, T.P., 1970. A facies analysis of the Arenig of western

Lleyn, North Wales. Proceedings of the Geologists'

Association, 81, pp. 221-239.

Dimberline, A.J., Bell, A. & Woodcock, N.H., 1990. A laminated

hemipelagic facies from the Wenlock and Ludlow of the

Welsh Basin. Journal of the Geological Society, London,

Vol.147, pp. 693-701.

Dix, E., 1933. The succession of fossil plants in the Millstone Grit

and the lower portion of the Coal Measures of the South

Wales Coalfield (near Swansea). Palaeontographica, 78B,

pp. 158-202.

Dixey, F. & Sibley, T.F., 1918. The Carboniferous Limestone Series

on the south-eastern margin of the south Wales Coalfield.


73, pp. 111-164.

Dixon, E.E.L., 1921. The geology of the South Wales Coalfield. Part

XIII. The country around Pembroke and Tenby. Memoir of

the Geological Survey of Great Britain.

Dixon, E.E.L. & Pringle, J., 1927. The 'Penlan Quartzite'. Summary

of the Progress of the Geological Survey of Great Britain

for 1926, pp. 123-126.

Dixon, E.E.L. & Vaughan, A., 1911. The Carboniferous Succession in

Gower (Glamorganshire), with notes on its fauna and

conditions of deposition. Quarterly Journal of the

Geological Society of London, 67, pp. 477-571.

Dunne, W.M., 1983. Tectonic evolution of SW Wales during the Upper

Palaeozoic. Journal of the Geological Society of London,

Vol. 140, pp. 257-265.

Elliott, T. & Lapido, K.O., 1981. Synsedimentary gravity slides

(growth faults in the coal measures of South Wales).

Nature, 291, pp. 220-222.

Evans, D.C., 1906. The Ordovician rocks of western Caermarthenshire.


62, pp. 597-643.

Evans, D.G. & Jones, R.G., 1929. Notes on the Millstone grit of the

North Crop of the South Wales Coalfield. Geological

Magazine, 66, pp. 164-177.

Fitton, J.G. & Hughes, D.J., 1970. Volcanism and plate tectonics in

the British Ordovician. Earth Planetary Science letters,

8, pp. 223-228.

Frodsham, K.G.J., 1990. An investigation of geological structure

within opencast coal sites in South Wales. Unpublished

PhD Thesis, University of Wales, Cardiff.

Frodsham, K., Gayer, R.A., James, J.E. & Pryce, R., 1991. Variscan

thrust deformation in the South Wales Coalfield: a case

study from Ffos Las opencast coal site. In: Gayer, R.A. &

Greiling, R.O. (eds.). The Rhenohercynian and

sub-Variscan Fold Belts. Earth Evolution Science Series,

Vieweg, Braunschweig.

Gayer, R.A., Cole, J., Frodsham, K., Hartley, A.J., Hillier, B.,

Miliorizos, M. & White, S.C., 1991. The role of fluids in

the evolution of the South Wales Coalfield foreland

basin. Proceedings of the Ussher Society, Vol. 7 (4),

CHAPTER FOUR



Page 4-73

pp. 380-384.

Gayer, R.A. & Jones, J., 1989. The Variscan foreland in South Wales.

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

George, T.N., 1927. The Carboniferous Limestone (Avonian) Succession

of a portion of the North Crop of the South Wales

Coalfield. Quarterly Journal of the Geological Society of

London, 83, pp. 38-93.

George, T.N., 1956a. Carboniferous Main Limestone of the East Crop

in South Wales. Quarterly Journal of the Geological

Society of London, 111, pp. 309-322.

George, T.N., 1956b. The Namurian Usk Anticline. Proceedings of the

Geologists' Association, 66, pp.297-316.

George, T.N., 1958. Lower Carboniferous palaeogeography of the

British Isles. Proceedings of the Yorkshire Geological

Society, 31, pp. 227-318.

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

Geology, HMSO.

George, T.N., 1974. Lower Carboniferous Rocks in Wales. In: The

Upper Palaeozoic and Post-Palaeozoic Rocks of Wales (ed.)

T.R. Owen. University of Wales Press, Cardiff.

Green, J.F.N., 1908. The geological structure of the St David's area

(Pembrokeshire). Quarterly Journal of the Geological


Hancock, P.L., Dunne, W.M. & Tringham, M.E., 1982. Variscan

structures in 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.

Hartley, A.J. & Gillespie, P.A., 1990. Controls on alluvial

architecture by synsedimentary faults in the Coal

Measures of South Wales. Geological Journal, 25,

pp. 189-197.

Hicks, H., 1884. On the Pre-Cambrian rocks of Pembrokeshire, with

especial reference to the St. David's District. With an

Appendix by Thomas Davies. Quarterly Journal of the

Geological Society of London, Vol. 40, pp. 507-560.

Hicks, H., 1877. On the Pre-Cambrian ( Dimetian and Pebidian) rocks

of St. David's. Quarterly Journal of the Geological


Hillier, B.V., 1992. Seismic studies of deep structure beneath the

South Wales Coalfield and adjacent areas. Unpublished PhD

Thesis, University of Wales, Cardiff.

Jones, D.G., 1974. The Namurian Series in South Wales. In: Upper

Palaeozoic and Post-Palaeozoic rocks of Wales (ed.) T.R.

Owen. University of Wales Press, Cardiff.

Jones, D.G. & Owen, T.R., 1966. The Millstone Grot Succession

between Brynmawr and Blorenge, South Wales, and its

relationship to the Carboniferous Limestone. Proceedings

of the Geologists' Association, 77, pp.187-198.

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., 1989. The influence of contemporaneous tectonic

activity on Westphalian sedimentation in the South Wales

Coalfield. In: Atherton, Gutheridge & Nolan (eds)

Devonian and Carboniferous tectonics and sedimentation.

Yorkshire Geological Society, Special Publication.

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

CHAPTER FOUR



Page 4-74

deformation of the South Wales Coalfield. Journal of the

Geological Society of London, Vol. 148, Part 5,

pp. 881-891.

Kelling, G., 1988. Silesian sedimentation and tectonics in the South

Wales basin: a brief review. In: Besly and Kelling (eds.)

Sedimentation in a synorogenic basin complex. The Upper

Carboniferous of North-west Europe.

Khalaf, F.I., 1980. Incipient metamorphic fabrics in the

Carboniferous rocks of the South Wales Coalfield. Journal

of the University of Kuwait (Sci.) 7, pp. 165-184.

King, W.W., 1925. Notes on the "Old Red Sandstone" of Shropshire.

Proceedings of the Geologists' Association, 36,

pp. 383-389.

King, W.W., 1934. The Downtonian and Dittonian strata of Great

Britain and North-western Europe. Quarterly Journal of

the Geological Society of London, 90, pp. 526-570.

Kokelaar, B.P., 1988. Tectonic controls of Ordovician arc and

marginal basin volcanism in Wales. Journal of the

Geological Society, London, 145, pp. 759-776.

Le Gall, B., 1991. Crustal evolutionary model fro the Variscides of

Ireland and Wales from SWAT seismic data. Journal of the

Geological Society, London, Vol. 148, pp. 759-774.

Leitch, D., Owen, T.R. & Jones, D.G., 1958. The basal Coal Measures

of South Wales Coalfield from Llandybie to Brynmawr.


113, pp. 461-486

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.

Moore, L.R., 1945. The geological sequence of the South Wales

Coalfield: the 'South Crop' and Caerphilly Basin and its

correlation with the Taff valley sequence. Proceedings of

South Wales Institute of Engineers, 60, pp. 141-227.

Moore, L.R., 1948. The sequence and structure of the southern

portion of the East Crop of the South Wales Coalfield.

Quarterly Journal of the Geological Society, London, 103,

pp. 261-294.

Owen, T.R., 1954. The structures of the Neath disturbance between

Bryniau Gleison and Glynneath, South Wales. Quarterly

Journal of the Geological Society, London, 109,

pp. 333-365.

Pick, M.C., 1964. The stratigraphy and sedimentary features of the

Old Red Sandstone, Portishead coastal section, north-east

Somerset. Proceedings of the Geologists' Association, 75,

pp. 199-221.

Potter, J.F. & Price, J.H., 1965. Comparative sections through rocks

of Ludlovian- Downtonian age in the Llandovery and

Llandeilo districts. Proceedings of the Geologists'


Powell, C.M., 1987. Inversion tectonics in SW Dyfed. Proceedings of

the Geologists' Association, 98, pp.193-203.

Pringle, J. & George, T.N., 1948. South Wales (2nd Edition), British

Regional Geology, HMSO.

Ramsbottom, W.H.C., 1970. Carboniferous faunas and palaeogeography

of the South-west England region. Proceedings of the

Ussher Society, 2, pp. 144-157.

Ramsbottom, W.H.C., 1973. Transgressions and regressions in the

Dinantian: a new synthesis of British Dinantian

CHAPTER FOUR



Page 4-75

stratigraphy. Proceedings of the Yorkshire Geological

Society, 39, pp. 567-607.

Ramsbottom, W.H.C., 1979. Rates of transgression and regression in

the Carboniferous of NW Europe. Journal of the Geological


Rast, N., 1969. The relationship between Ordovician structure and

volcanicity in Wales. In: The Pre-Cambrian and Lower

Palaeozoic rocks of Wales, (Ed.) Alan Wood, Cardiff,

University of Wales Press

Palaeozoic Rocks of Wales, (ed.) Alan Wood, Cardiff,

University of Wales Press.

Richardson, J.B. & Rasul, S.M., 1990. Palynofacies in a Late

Silurian regressive sequence in the Welsh Borderland and

Wales. Journal of the Geological Society of London, Vol.

147, part 4, pp.675-686.

Robertson, T., 1927. The Geology of the South Wales Coalfield Part

II: Abergavenny. Memoir of the Geological Survey of

England and Wales.

Robertson, T. & George, T.N., 1929. The Carboniferous Limestone of

the North Crop of the South Wales Coalfield. Proceedings

of the Geologists' Association, London, 49, pp. 18-40.

Sanzen-Baker, I., 1972. Stratigraphical relationships and sedimentary

environments of the Silurian - early Old Red Sandstone of

Pembrokeshire. Proceedings of the Geologists'


Squirrel, H.C. & Downing, R.A., 1969. Geology of the South Wales

Coalfield, Part I. The country around Newport. Memoir of

the Geological Survey of Great Britain.

Strachan, A., 1899. The Geology of the South Wales Coalfield. Part

I: The Country around Newport, Monmouthshire (1st

Edition). Memoir of the Geological Survey of Great

Britain.

Strachan, A., 1909. The Geology of the South Wales Coalfield. Part

I: The country around Newport (2nd Edition). Memoir of

the Geological Survey of England and Wales.

Strachan, A., Cantrill, T.C., Dixon, E.E.L., Thomas, H.H. & Jones,

O.T., 1914. The Geology of the South Wales Coalfield,

Part XI. The country around Haverfordwest. Memoir of the

Geological Survey of England and Wales.

Straw, S.H., 1930. The Siluro-Devonian boundary in south-central

Wales. Journal of the Manchester Geologists' Association,

1, pp. 79-102.

Straw, S.H., 1953. The Silurian succession at Cwm Craig Ddu

(Breconshire). Liverpool Manchester Geological Journal,

1, pp. 208-219.

Sullivan, R., 1964. The Lower Carboniferous rocks of the Castle Hill

Fault Block, Tenby, Pembrokeshire. Geological Magazine,

Vol. 101, No.2, pp. 113-115.

Thomas, L.P., 1974. The Westphalian (Coal Measures) in South Wales.

In: Upper Palaeozoic and Post-Palaeozoic rocks of Wales,

(ed.) T.R. Owen, University of Wales Press, Cardiff,

pp.133-160.

Thorpe, R.S., 1972. Possible subduction origin for two Pre-Cambrian

calc-alkaline plutonic complexes from southern Britain.

Bulletin of the Geological Society of America, 83,

pp. 3663.

Traynor, J.J., 1988. The Arenig in South Wales: sedimentary and

volcanic processes during the initiation of a marginal

basin. Geological Journal, 23, pp. 275-292.

CHAPTER FOUR



Page 4-76

Trueman, A.E., 1947. Stratigraphical problems in the coalfields of

Great Britain (presidential address). Quarterly Journal

of the Geological Society of London, 103, LXV-CIV.

Tunbridge, I.P., 1986. Mid-Devonian tectonics and sedimentation in

the Bristol Channel area. Journal of the Geological

Society, London, 143, pp. 107-116.

Vaughan, A., 1905. The palaeontological sequence in the

Carboniferous Limestone of the Bristol area. Quarterly

Journal of the Geological Society of London, 61,

pp. 181-307.

Walmsley, V.G. & Bassett, M.G., 1976. Biostratigraphy and

correlation of the Coralliferous Group and Gray Sandstone

Group (Silurian) of Pembrokeshire, Wales. Proceedings of

the Geologists' Association, 87, pp. 191-220.

Ware, W.D., 1939. The Millstone Grit of Carmarthenshire. Proceedings

of the Geologists' Association, London, 50, pp. 168-204.

Williams, B.P.J., 1971. Sedimentary features of the Old Red

Sandstone and Lower Limestone Shales of south

Pembrokeshire, south of the Ritec Fault. pp. 222-239. In:

Bassett, D.A. & Bassett, M.G. (eds.), Geological

Excursions in South Wales and the Forest of Dean.

Geologists' Association, South Wales Group, Cardiff.

Williams, A., Strachan, I., Bassett, D.A., Dean, W.T., Ingham, J.K.,

Wright, A.D. & Whittington, H.B., 1972. A correlation of

Ordovician rocks in the British Isles. Geological Society

Special Report, no. 3.

Wilson, D., Davies, J.R., Fletcher, C.J.N. & Smith, M., 1990.

Geology of the South Wales Coalfield, Part VI, the

country around Bridgend, Memoir for 1:50,000 geological

sheet 261 and 262 (England and Wales). Second edition,

London: HMSO.

Woodcock, N.H., 1984. Early Palaeozoic sedimentation and tectonics

in Wales. Proceedings of the Geologists' Association, 95,

pp. 322-335.

Woodcock, N.H., 1988. Strike-slip faulting along the Church Stretton

Lineament, Old Radnor Inlier, Wales, Journal of the

Geological Society, London, Vol. 145, pp. 925-933.

Woodcock, N.H., 1990. Sequence stratigraphy of the Welsh Basin.

Journal of the Geological Society, London, 147,

pp. 537-547.

Woodcock, N.H. & Gibbons, W., 1988. Is the Welsh Borderland Fault

System a terrane boundary? Journal of the Geological

Society of London, Vol.145, pp. 915-923.

Woodland, A.W., & Evans, W.B., 1964. The Geology of the South Wales

Coalfield, Part IV. The country around Pontypridd and

Maesteg. Memoir of the Geological Survey of Great

Britain.

Woodland, A.W., Evans. W.B. & Stephens, J.V., 1957. Classification

of the Coal Measures of South Wales with special

reference to the Upper Coal Measures. Bulletin of the

Geological Survey of Great Britain, 13, pp. 6-13.

Ziegler, A.M., McKerrow, W.S., Burne, R.V. & Baker, P.E., 1969.

Correlation and environmental setting of the Skomer

Volcanic Group, Pembrokeshire. Proceedings of the

Geologists' Association, 80, pp. 409-439.

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FIGURE CAPTIONS

Fig. 4.1 Isopach map of the Namurian sequence (George, 1970) showing contrasting distributions of thickness in SW Dyfed

and the main coalfield.

Fig. 4.2 Structural sketch map of the St David's area showing general structural trends and hypothetical directions of slip

along the main faults. (Based on Green, 1908).

Fig. 4.3 Prolific conjugate tension gashes found at Fishguard, showing the same structural trend as conjugate faults of North

Devon of Variscan age.

Fig. 4.4 Deviation of Caledonian structural trend from NE-SW, as found in Mid Wales, to ENE-WSW as in SW Dyfed.

(Figure from Rast, 1969).

Fig. 4.5 Hypothetical movement history for anomalously trending disturbances. Key: Ex, Extension; Co, Compression;

Horizontal shift in plot indicates inversion has taken place; Vertical shift indicates reactivation. Vertical axis, time.

Fig. 4.6 Regional location of the South Wales Coal Basin. Key to coalfields: P, Pembrokeshire; SW, South Wales; FoD,

Forest of Dean; SB, Somerset and Bristol; OB, Oxfordshire and Berkshire; K, Kent; N, Namur; L, Liège; A, Aachen; R,

Ruhr. Key to map: 1, coal bearing foreland basins; 2, Lower Palaeozoic massifs; 3, major Variscan thrusts marking the

hinterland limits of the foreland basins; 4, hypothetical southern margin of the Dinantian carbonate platform.

Fig. 4.7 Chronostratigraphic diagram emphasising the contrast in Devonian stratigraphy in S Wales and N Devon (Allen,

1974).

Fig. 4.8 Correlation chart for Old Red Sandstone formations in Wales and the Welsh Borderlands by (Allen, 1974) showing

the regional nature of the Mid Devonian unconformity.

Fig. 4.9 Synoptic sketch map and section of 'Acadian' structures north of the Carreg Cennen Disturbance (Cope, 1979).

Stratigraphic key: Pc, Precambrian; C, Cambrian; T, Tremadocian; A, Arenigian; Ll, Llandoverian; ors, Old Red Sandstone;

D, Dolerite.

Fig. 4.10 Hypothetical movement histories for the Carreg Cennen Fault. Key: Ex, Extension; Co, Compression.

a. Mid Devonian compression associated with Late Caledonian tectonism.

b. Mid Devonian compression associated with earliest Variscan tectonism.

c. Mid Devonian quiescence.

d. Separate Mid Devonian Acadian compressional event.

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Fig. 4.11 Map of the northerly derivation of the Namurian of the South Wales Coalfield (George,1970) with a recent version

by Hartley showing local southerly sources.

Fig. 4.12 Early isopach maps for the Coal Measures of the South Wales Coalfield by Thomas (1974).

Fig. 4.13 Map showing the southerly derivation of sediment during Late Westphalian times (Kelling, 1974) due to uplift in

the hinterland and subsequent loading and infill of the foreland basin.

Fig. 4.14 Recent isopach maps for the Coal Measures of the South Wales Coalfield by Hartley & Hillier (1992).

Fig. 4.15 a. Location of Llanstephan in South Wales and of the coastal section studied. Key: CCD, Carreg Cennen

Disturbance; Cs, Carboniferous; Dv Devonian; Sn, Silurian; Ov, Ordovician.

b. Structural sketch map of the Carmarthen area based on BGS 1:50,000 series, Carmarthen sheet.

Fig. 4.16 North and south verging thrusts of metre-decametre scale lengths along the Llanstephan coastal section.

Southwards verging metre-scale thrusts face gently downwards towards the south.

Fig. 4.17 A variety of lineation trends: north verging, downward facing folded thrusts with associated steeply plunging

slickenside lineations and sub horizontal thrust planes with N-S trending sub horizontally plunging lineations.

Fig. 4.18 E-W trending tension gash arrays indicating dextral shear.

Fig. 4.19 A likely kinematic diagram showing a NNW-SSE trending σ1 causing N-S thrusting and E-W shear due to NE-

SW strike-slip reactivation of the CCD. Key: σ1, maximum principal compressive stress; CCD, Carreg Cennen Disturbance.

Fig. 4.20 Structural sketch map of Llanstephan and stereographic projection of the structure. The accompanying photograph

shows the open geometry of the fold (near its crest).

Fig. 4.21 Late, mesoscale, south verging, back thrusts.

Fig. 4.22 Hypothetical pre- and post-fold thrust geometries in relation to bedding as a basis for interpreting the Llanstephan

structural section.

Fig. 4.23 a. Hypothetical kinematic diagram showing the minor E-W dextral shear brought about by sinistral strike-slip on

the CCD and N-S thrusting due to a N-S trending σ1. Key: σ1 maximum principal compressive stress; CCD, Carreg Cennen

Disturbance.

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b. Simple kinematic diagram showing the compartmentalisation of σ1 (N-S) into σn (NNW-SSE) and τ (ENE-

WSW) to form the NE trending folding observed on the section. (nb Further measurements are required to support the ENE

dextral shear predicted by this model).

Fig. 4.24 Sketch map of the South Wales Coalfield. Key: small circular ornament, Devonian ORS; block-work ornament,

Carboniferous Limestone; speckled grey ornament, Millstone Grit; plain white ornament, Lower and Middle Coal Measures;

tightly packed pebble ornament, Pennant Measures. Towns: A, Aberdare; S, Swansea. Opencast coal sites: Ps, Park Slip

Extension; FfL, Ffos Las; GI, Gilfach Iago; Gn, Garnant; EP, East Pit Extension; NH, Nant Helen; D, Derlwyn; FfA,

Ffyndaff Additional; PBO, Pen Bryn Oer; BF, Benward's Field.

Fig. 4.25 Reactivation history of major faults with various structural trends in the South Wales area.

Fig. 4.26 a-d. Developed version of a section through Park Slip Extension (based on Jones, 1989) showing seam splitting,

back thrusting and decametre-scale complex folding and thrusting as illustrated in the accompanying photograph.

Fig. 4.27 Clear examples of synsedimentary extension in Park Slip Extension, (though larger scale thickness variations may

be due to later Variscan tectonism).

Fig. 4.28 Hypothetical structural history plot for SW Britain tentatively correlating similar types of deformation phases, eg

reactivation dominated or main phase thrust-dominated phases, as a means of showing the local structural effects of an

advancing orogenic front through time.

Fig. 4.29 a. Structural section of the southern part of the eastern high wall of Ffos Las (Gayer pers. com., 1992) showing

about 70% shortening of the Big Rock Sandstone. Accessible areas have been outlined in this figure and illustrated by the

accompanying photographs of Fig. 4.29b.

Fig. 4.29 c. The stratigraphy of Ffos Las (Frodsham, 1990 after R. Thewlis - British Coal S Wales coal seam correlation

chart).

Fig. 4.30 Synoptic sketch of the east face of Ffos Las OCCS and accompanying photograph to illustrate the general structure

further.

Fig. 4.31 a. Synoptic section showing the folded nature of thrusts in the core of the

folds at Ffos Las.

b. Diagram of a tightly folded thrust plane and photograph showing examples

of other tight folds and folded thrusts at Ffos Las.

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Fig. 4.32 Structural details of a repetition of the Big Vein seam showing the development of cleat perpendicular to bedding

(B) and a cleavage duplex (Cl) associated with intra-seam shearing.

Fig. 4.33 Folding associated with the Big C Vein.

Fig. 4.34 North verging thrusts at high levels in the Ffos Las site and a sketch of north directed folding and thrusting in the

Green and Kings seams.

Fig. 4.35 Sketch section and accompanying photograph of the main structure observed in the lower part of the east face of

Ffos Las located in Fig. 4.29a. Key: sst, sandstone; sh, shale; c, coal.

Fig. 4.36 Sketch map of the Gower Peninsula showing the general structure and localities containing good examples of

thrusts: CB, Caswell Bay; PEyB, Port Eynon Bay; RhB, Rhossili Bay.

Fig. 4.37 Early section of the high wall at Ffyndaff Additional displaying an extensional fault displacing the 6ft and 4ft

seams.

Fig. 4.38 NE to ENE facing tight rounded fold above the roof of the Upper 6ft. The facing direction is anomalous for this

site.

Fig. 4.39 Oblique section through the 2ft 9" seam which is thickened by north verging thrusts. A distinct lens-shaped

outcrop is evident in the 2ft 9" seam.

Fig. 4.40 Stereographic projection of a bedding plane containing the pyrite strain fabric. The fabric indicates a probable

dextral shear along its pitch upon the bedding surface.

Fig. 4.41 a. Outline of a linked northward transporting thrust system interpreted from the high wall of Ffyndaff Additional.

Key: 1-2-3-4 One possible explanation of the sequence of thrusting; Shc, sheared shaley coal horizon.

b. Panoramic view of the high wall at Ffyndaff Additional with excavation taking place at Lower 6ft levels. The

view shows the basis for Fig. 4.41a.

Fig. 4.42 a. Section through the 6ft seam showing thrust repetition by short ramps and long flats exploiting the weakest

horizons.

b. Photographs illustrating the Lower and Upper 6ft seams and fractures and cleavage within the Lower 6ft seam as

a possible indication of layer parallel shearing near the thrust.

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Fig. 4.43 a. Repetition of the 6ft seam in to Lower and Upper seams with further probable thrust repetition of the Upper 6ft

seam at Ffyndaff Additional. The accompanying photograph illustrates a pop-up structure associated with back thrusting at

Upper 6ft levels.

Fig. 4.44 Schematic cross section through Ffyndaff Additional OCCS illustrating a hypothetical case of out-of-sequence

thrusting and lesser in-sequence thrusting at 6ft levels.

Fig. 4.45 Latest, developed section from a mature high wall of Ffyndaff Additional showing the concentration of thrusting at

6ft levels, below the 4ft seam.

Fig. 4.46 Accommodation structures at Gilfach Iago.

a. Thrusting developed in the core of the syncline ("a valley of coal - mined at Gilfach Iago").

b. Sketch of the syncline and details of the accommodation structure.

Fig. 4.46 c. Thrusting in the Trichwart seam, unrelated to accommodation.

Fig. 4.47 Sketch and photograph of the major anticline close to the Caer Bryn Disturbance (Glyn-Hir Anticline).

Fig. 4.48 Map and kinematics diagram to show the compartmentalisation of the σ1 maximum principal compressive stress

into σn normal stress and τ shear stress to form the Glyn Hir Anticline (GHA). Key continued: CFF, Cwm Fferws Fault; MP,

Middle Pumpquart; S, Stanllyd.

Fig. 4.49 Sketch and photograph of decametre length northward directed folded thrust as evidence for early thrusting.

Fig. 4.50 Sketch and photograph of late thrusting (a duplex) cutting into the north-western limb of the anticline.

Fig. 4.51 Sketch and photograph of the ENE facing monocline at East Pit Extension. The monocline is a high level

expression of a west dipping fault which is conjugate to the buried east dipping Cwm Teg Fault.

Fig. 4.52 Listric extensional faults at East Pit Extension OCCS represented by line drawings in Fig. 4.51.

Fig. 4.53 a. Recent exposures at deep levels in East Pit Extension showing clear evidence for NNE directed thrusting

affecting the Big seam and Harnlo seam. Thrusting climbs up sequence northwards though it is unclear as to whether the

compressional structure in 4.53b below is directly related.

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Fig. 4.54 1:10,000 scale geological maps of selected areas along the Cribarth Disturbance showing the following:

a. The NE-SW trending major fold deforming the Carboniferous Limestone and Millstone Grit across the SW

extent of the Cribarth Disturbance.

b. Map of the NE extent of the Cribarth Disturbance where folded Carboniferous Limestone CMOo & SOol is

faulted against Devonian ORS to the NW.

c. Map of a river section through the most north-eastern extent of the Cribarth Disturbance within Lower and

Upper Old Red Sandstone showing a change in strike across the probable NE extent of the Swansea Valley Fault Zone.

Key: ORS, Devonian Old Red Sandstone. Visean Limestone: CMOo, Calcareous Mudstone and Oolite Formation; SOol,

Skeletal Oolite Formation; CM, Calcareous Mudstone Unit; BCP, Black Cherty Packstone Formation; SOo & CP, Skeletal

Oolite and Cherty Packstone; CQW, Calcareous Quartz Wacke Unit. Upper Carboniferous: MG, Millstone Grit quartz

conglomerate.

Key to map symbols: 1. geological boundary; 2. fold axial trace; 3. dip measurement; 4. minor strike-slip fault; 5. minor dip-

slip fault; 6. cleavage measurement; 7. major fault line.

Fig. 4.55 a. Sketch map of the major Caledonoid Disturbances in South Wales showing the compartmentalisation of an early

Variscan stress system (after Weaver, 1975).

b. Kinematics diagram relating the small-scale structure in the Cribarth area to the compartmentalised stress model

of Weaver (1975). Key: CCD, Carreg Cennen Disturbance; SVD, Swansea Valley Disturbance; NVD, Neath Valley

Disturbance. Vn, E-W extensional vein; SS, slickenside lineations; σ1, maximum principal compressive stress; τ, shear

stress; σn, normal stress.

c. Stereographic projection of the Cribarth structure to qualify the kinematics diagram and possibly Weaver's

model.

Fig. 4.56 Photograph and sketch of the Pwllau Bach Fault (PBF) at Nant Helen, with details of thrusting and possible

synsedimentary faulting in the hangingwall of the PBF.

Fig. 4.57 Thrusting at Nant Helen ramping from Harnlo levels to White Levels with intense layer parallel shearing in the

Black seam. Details of small scale ramps affecting White 4ft to Stwrin levels are given in the Upper part of the section. The

Pwllau Bach Fault downthrows 80m to the west, faulting the White 4ft against the Brass to the east.

Fig. 4.58 Extensional cross faulting bounding the Derlwyn site, displacing the Bluers against the Gnapiog.

Fig. 4.59 Photograph of the Amman Marine Band above the Meadow seam at Benward's Field OCCS.

Fig. 4.60 Sketch map of a part of the Benward's Field OCCS showing NE-SW trending folding possibly related to

synsedimentary deformation or related to a combination of thrusting and lag faulting.

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Fig. 4.61 Sketch and detailed photograph of a section of an extensional fault at Benward's Field with minor faults and

bedding blocks arranged in an array along the fault plane.

Fig. 4.62 Kinematic models to explain the orientation of bedding blocks and minor faults in relation to normal faulting. Key:

Rl Sh, Riedel Shear.

Fig. 4.63 Extensional fault showing low angle of dip and slip located along the coal seam base at Benward's Field.

Fig. 4.64 An unclear case as to whether sandstone thickening occurred across a section of a fold due to synsedimentary

folding and faulting or due to post-depositional tectonic repetition.

Fig. 4.65 a. Thrust repetition of the Harnlo seam at Garnant, with thrusts verging towards the north-east.

b. Developed and mature high wall of Garnant showing a clear section of thrust repeated channel sandstone beds

also showing sedimentological lateral variations in bedding thickness.

c. Photograph of the Black seam showing a sheared top layer and intact lower layer. A sketch shows the reverse

case where chevron folds containing pyrite saddle reefs overlie a cleavage duplex. The lower photograph shows further

intraseam thrusting and folding.

Fig. 4.66 Stacked sandy siltstone-filled channels with no apparent cross bedding structure, located between the Big seam (6-

4ft seams) and the Elled seam (2ft 9" seam). The channels occur above a cross fault.

Fig. 4.67 Scissors fault affecting the Big Vein showing an anticlockwise torsional movement on looking westwards.

Displacement is on a metre scale.

Fig. 4.68 a. Model for a basal thrust climbing from beneath the South Wales Coalfield and in to the Middle Coal Measures.

A sole thrust may climb up a basement fault and reach the Middle Coal Measures due to the shallow depth to basement

further north (Hillier, 1992).

b. An alternative case in which a thrust climbs up a gentle basement incline as a flat and continues to climb as a

ramp, at the same angle as the incline below, into Middle Coal Measures. This occurs because the beds above the incline

would lose their dip on passing northwards onto the basement high.

Fig. 4.69 Structural sketch map of South Wales showing a practical subdivision of the South Wales Coalfields into structural

environments A-F.

Fig. 4.70 Sketches of structures observed in SW Dyfed Pembrokeshire Coalfield which emphasise the complexity of folding

and thrusting and the similarity of the structure of SW Dyfed with that of the western part of the main coalfield. Sketches are

of structures from Little Haven, Broad Haven, Saundersfoot, Amroth and Monkstone Point.

Marios Miliorizos 11th June 2006 File name PhD Chapter 4 Four

tectonic evolution of the bristol channel borderlands chapter 4

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