tectonic evolution of the bristol channel borderlands chapter 5

CHAPTER FIVE

Tectonic Evolution of the Bristol Channel borderlands

SEISMIC INTERPRETATION, VALE OF GLAMORGAN

Page 5-0

‘The clotted vibrancy of spring no longer

overflows with my spirit. It slips sparsely

and powerless into the convoluted locks of hair

of the lost correspondent. The wind dries it up,

the sea remains uncured. It will never meet with

the sick bees, it no longer soaks my handkerchief,

it does not tolerate the inexpressible force for

new life.’

Based on Crude Oil by:

NIKOS ALEXIS ASLANOGLOU

and its translation by:

M.B. RAIZIS.

CHAPTER FIVE


SEISMIC INTERPRETATION, VALE OF GLAMORGAN Page 5-1

5. GEOLOGICAL INTERPRETATION OF SEISMIC REFLECTION SECTIONS

FROM THE VALE OF GLAMORGAN, SOUTH WALES.

5.1 AIMS

The aims of the chapter are: (1) to investigate the pre-Mesozoic stratigraphy of the Vale of

Glamorgan using seismic reflection sections, complemented by seismic refraction data; (2) to

identify the main geological structures displayed on the reflection sections and to interpret

them using the geological control provided by local Palaeozoic inliers; and (3) to use the

structural data to deduce a regional structural evolution and to investigate the occurrence of

any basement-related, fault reactivation events.

5.2 INTRODUCTION

Three reflection lines were shot in the Vale of Glamorgan by Shell UK Expro during August

1989 who has kindly made the data available to the current research project. The data consist

of two north-south dip sections (SG89-01V and SG89-02V) and an east-west strike section

(SG89-03V). The dip sections are about 22km long and extend from the coast of the Vale

northwards to the south crop of the coalfield. The strike section is about 16km in length and

ties the southern ends of the dip sections (Fig. 5.1).

The reflection data were acquired using a Vibroseis source (this equipment and vehicles are

described by Dobrin & Savit (1988)). The energy source consisted of four vibrators at 10m

separation.

CHAPTER FIVE



The geophone array length was 23.92m consisting of 2 strings of 12 geophones in line at a

spacing of 1.04m. The fold of coverage was a nominal 60 fold. Digital data recording tapes as

well as complete filtered stack and migrated reflection sections were provided for analysis.

Geophysical investigation of the seismic data was carried out by Hillier (1992) for the purpose

of analysing potential basement events beneath the Vale of Glamorgan as part of a PhD

project. Geological analysis of the general structure was undertaken by the author. The

seismic data have thus been incorporated into two related structural topics: the first correlating

the reflection data with regional-scale refraction profiles in South Wales and the second

correlating the reflection data with detailed onshore geological surveys. The two analyses

were intended to assist in establishing the regional stratigraphy and structure and to contribute

to an evolutionary model of the geology of the Bristol Channel Borderlands.

The seismic reflection data (hereinafter, Vale reflection lines) image the Variscan structure

concealed by the veneer of Mesozoic strata and, in terms of subsurface geology, are also

important in providing a link with the reflection data from the Bristol Channel (discussed in

Chapter 6).

5.3 METHODOLOGY

Initially, depth conversion of selected reflection events was carried out using interval and

stacking velocity information and a GWBASIC program (Appendix 5.2) to assist in their

geological identification. Depth-converted reflection events were also compared with refractor

depths derived from complementary wide-angle quarry blast and marine seismic lines (Hillier,

1992).

Secondly, line drawings of the geometrical configuration of prominent reflection events

(marker horizons) were produced. Groups of reflectors were then organised into seismic

packages, including cross cutting features, for the purpose of seismic mapping, i.e. correlation

of the main events and packages across the sections.

Other methods of seismic interpretation were also utilised e.g. (Fitch, 1976; Anstey, 1977;

Brown & Fisher, 1980; Kleyn, 1982; Sheriff, 1982; McQuillin et al, 1984; Jenyon & Fitch,

1985).

The results obtained from seismic interpretation were used to produce various geological

models compatible with the surface geology.

CHAPTER FIVE



5.4 PRE-MESOZOIC STRATIGRAPHY OF THE VALE OF GLAMORGAN AND

SURROUNDING AREAS.

This chapter presents a geological interpretation of the Vale reflection lines. The geological

controls for interpretation of the reflection data are (a) inliers of Palaeozoic rocks in the Vale;

(b) deep boreholes; and (c) relevant outcrops in surrounding areas.

The exposed pre-Mesozoic sequence in the Vale of Glamorgan is at least 2km thick,

consisting of an unknown thickness of Silurian, at least 1km of Devonian and about 1km of

Carboniferous Limestone. It is likely that Upper Carboniferous strata are also concealed

beneath the Mesozoic cover of Penmark so that 2km is certainly a minimum value. However,

the total thickness of seismically layered, sub-Mesozoic sequence on line SG89-02V, beneath

shot point 500 and above anticipated basement, is about 6.5km. Clearly there is about 4.5km

of unexposed layered sequence beneath the Vale of Glamorgan. Different possibilities to

explain the composition of the entire sequence to basement depths are presented below after a

systematic correlation of seismic packages on lines SG89-01V to 03V with the general

stratigraphy of the Vale of Glamorgan and adjacent areas.

There is a clear problem in interpreting the Vale reflection lines in that 4.5km of sequence

remains undefined. However borehole data of Cambrian Exploration Ltd extend local

stratigraphic information down to the Ordovician. They revealed a Silurian thickness of 360-

390m and also evidence for an unknown thickness of Ordovician volcanics (Fig. 5.2).

CHAPTER FIVE



The only other information on deep structure is upper crustal models based on large scale

refraction/wide angle surveys (Mechie & Brooks, 1984). These defined a deep basement

surface at about 6km (on Line J) succeeded by a Precambrian suprabasement/Lower

Palaeozoic layer about 3-4km thick (extending from a depth of about 2-3km to 6km depth).

These albeit approximate depth values correlate with the 2km and 6.5km depth estimates to

the top of the Lower Palaeozoic and the base of the seismically layered sequence, interpreted

here as the basement surface.

If the above correlation is correct, the Vale reflection lines confirm the presence of a thick

Precambrian suprabasement/Lower Palaeozoic layer which can now be said to be layered,

composite and to contain a number of discordances. Surrounding regions are now considered

for possible analogous rocks at outcrop.

The following areas were studied: Johnston, St David's and Carmarthen (Precambrian); SW

Dyfed, Builth Wells, Malverns, Usk, Bristol district and Rumney (Lower Palaeozoic); SW

Dyfed and Vale of Glamorgan (Devonian); Gower Peninsula, Vale of Glamorgan and Taff's

Well (Carboniferous Limestone); South Wales Coalfield - south crop (Upper Carboniferous).

5.4.1 PRECAMBRIAN AND LOWER PALAEOZOIC

SW Dyfed contains major outcrops of Precambrian crystalline basement and volcanics (Baker,

1971, 1982) and Lower Palaeozoic marginal basin sediments (Woodcock, 1990). These

geological elements of SW Dyfed are likely to be represented in the deep structure of central

South Wales. In particular, the Vale of Glamorgan is likely to be underlain by a Precambrian

and Lower Paleozoic sequence more akin to the sequence cropping out in SW Dyfed than to

the condensed sequence of the Welsh Borderlands.

Precambrian Basement

The major refracting boundary identified by Mechie & Brooks (1984) at a depth of <1km in

the St David's area and at 4km in the south below St Govan's Head (Fig. 5.3a) was correlated

locally with basement rock which crops out to form the Johnston Block (elevated to its

structural position by the Johnston Thrust (Dunne, 1983) and regionally with a widespread

basal refractor beneath South Wales.

The Johnston Block consists of quartz diorites, quartz albite intermediates and quartz dolerites

(Strachan et al, 1914; BGS 1:50 000 sheet 228, Haverfordwest) and has been observed to

structurally overlie Coal Measure strata at e.g. Ticklas Point (Fig. 5.3b). Similar crystalline

rocks may thus form the basement beneath the Vale of Glamorgan.

CHAPTER FIVE



Precambrian layered series

The Dimetian and Pebidian layered series of SW Dyfed was first described by Hicks (1877,

1884). Blake (1884) subdivided the series into two volcanic suites: a lower trachytic series

succeeded by a rhyolitic series. Basic intrusives and minor volcanics in the complex were

thought by Blake to mark late and separate phases of igneous activity. Recent trace element

geochemical investigations by Thorpe (1972) proved an association of this igneous complex

with active plate margin processes. The Dimetian and Pebidian volcanics, being part of this

complex, therefore represent the oldest record of regional tectonism in the Bristol Channel

Borderlands.

It is likely that the Precambrian layered series is not restricted to SW Dyfed, and it could

underlie the Vale of Glamorgan in a similar fashion to the regional crystalline basement

surface of Bayerly & Brooks (1980) and Mechie & Brooks (1984) unless their extent is

limited by major 'Caledonoid' lineaments. Further evidence for the continuation to the east of

a Precambrian layered sequence is the occurrence of Precambrian sedimentary rocks in

Carmarthen (Cope, in press) as well as the Precambrian volcanics of the Midland Platform.

Such layered sequences of the volcanic series would be expected to generate a prominent

reflector package on seismic reflection sections.

Lower Palaeozoic

The Lower Palaeozoic sequence of SW Dyfed represents a back-arc basin succession along

the southern flank of the Welsh Basin (Anderton et al, 1979; Woodcock, 1984, 1990;

Kokelaar, 1988; Dimberline et al, 1990) unlike the marginal shelf setting of the Lower

Palaeozoic strata of the Welsh Borderlands.

The sequence consists of transgressive, intrashelf marine Cambrian facies (Green, 1908;

Anderton et al, 1979) extending to N Wales (Crimes, 1970) and on the platform to the east of

the Welsh Basin; an Ordovician benthic and intrashelf marine facies (Evans, 1906; Green,

1908; Williams et al, 1972; Traynor, 1988) and volcanic facies (Ziegler et al, 1969; Fitton &

Hughes, 1970; Traynor, 1988); a Silurian intrashelf marine regressive facies (Walmsley &

Bassett, 1976) extending into the Welsh Borderlands (Richardson & Rasul, 1990) and Siluro-

Devonian marine-nonmarine transitional facies (Allen et al, 1976; Allen & Williams, 1978).

From seismic refraction studies, Mechie & Brooks (1984) have postulated the occurrence of a

2-3km thick succession of Lower Palaeozoic strata beneath central South Wales. A thick

succession occurs in SW Dyfed so that this area represents a good onshore analogue.

CHAPTER FIVE



In support of this inference, the following sequences of SW Dyfed would be expected to

contain good seismic reflective horizons: the Upper Ordovician - Lower Silurian Skomer

Volcanic Series; the Llandeilo Limestone; the Silurian Robeston Wathen Limestone and

Shoalshook Limestone; the Wenlock and Ludlow Coralliferous Series and Sandstone Series.

Other Ordovician and Silurian sequences nearby are the series of Middle Ordovician grey-

green tuffs and rhyolites of Builth Wells and the sequence in the Silurian inlier of Cardiff

(Anderson, 1971) consisting of an unknown thickness of Ludlovian Rumney Grits (indurated,

fine grained, lithic-quartz arenites and wackes) and thinly-bedded, fossiliferous, calcareous-

quartz wackes.

It is also equally possible that the reflective sub-Devonian sequences represented on the Vale

seismic sections result from highly-indurated, layered sequences similar to those described

above i.e. generally Ordovician Volcanics or a condensed Silurian sequence resting upon a

thick Cambrian sequence. However there is a problem in explaining the entire thickness of

imaged layered sequence if a condensed sequence is considered.

Further areas of relevance to the deep geology of the Vale include the Lower Palaeozoic

inliers of the Bristol District e.g. the Tortworth Inlier. If the present thickness of layered

sequence beneath the Vale of Glamorgan represents a tectonic thickness the condensed

sequence of Bristol would also become a suitable analogue. Of particular interest in such a

case would be the unconformity between Llandoverian Lower Trap basaltic lavas, sandstone

and shales (about 500m) and the Tremadocian Micklewood Bed shales (at least 150m). A

similar unconformity occurs in the Malvern Hills (Brooks, 1970) indicating that the absence

of most of the Ordovician is a regional phenomenon and could characterise the Vale of

Glamorgan. If the unconformity occurs beneath the Vale its depth can best be estimated from

the thickness of Silurian strata of the Usk inlier.

5.4.2 UPPER PALAEOZOIC

Devonian

The Devonian sequence in the Cardiff District consists of the Lower Old Red Sandstone Red

Marls (800m), Llanishen Conglomerate (130), Brownstones (160m) and the Upper Old Red

Sandstone Quartz Conglomerate (40m) and has a minimum stratigraphic thickness of 1km.

Emphasis is placed here on the limited thickness of Upper Devonian. Devonian strata should

therefore account for the first kilometre beneath the Mesozoic veneer in the eastern part of the

Vale.

CHAPTER FIVE



Lithic components of coarse grained lithologies provide further evidence of the sub-Devonian

geology of the Bristol Channel area in addition to the direct evidence from the Silurian inliers

of Rumney, Usk and the Bristol district. Evidence for the type of sub-Devonian sequence

comes from the composition of clasts in Lower-Middle Devonian conglomerates exposed in

SW Dyfed and the Vale of Glamorgan e.g. the Ridgeway Conglomerate and the Llanishen

Conglomerate. The sedimentary lithic clasts are probably derived from Lower Devonian

sediments and to a lesser extent from Lower Palaeozoic. Crystalline lithic clasts are derived

from basement rocks, beneath the Bristol Channel (Williams, 1971), uplifted during Mid

Devonian times to form the Bristol Channel Landmass (Tunbridge, 1986). It is also possible

that some clasts were derived from local fault blocks such as the uplifted block associated

with the Ritec Fault in SW Dyfed and the separate Cowbridge Lineament in the Vale of

Glamorgan.

A test of the relative importance of local source areas in relation to the Bristol Channel

Landmass and, thus, of determining the sub-Devonian succession beneath the Vale of

Glamorgan is to compare the clast compositions of the Ridgeway conglomerate and the

Llanishen conglomerate. Such a study could be extended with potentially beneficial results to

Middle Devonian formations of North Devon (after e.g. Tunbridge, 1980) and to the Lower

and Upper Devonian of the Mendips at Portishead Bed levels.

Lower Carboniferous

Exposures of Carboniferous Limestone were examined along the west coast of the Vale of

Glamorgan and, inland, in quarries and road sections to obtain stratigraphic information

directly relevant to the Vale reflection lines. Results were also obtained from examination of

limestone on the Gower Peninsula and Taff's Well (section 5.5). Further stratigraphic

information of the Carboniferous Limestone was obtained from 10cm diameter BGS half-core

from Ogmore (Beacons Down Borehole) and from the recent BGS 1:50 000 series sheet

261/262 Bridgend. Selected BGS 1:10 000 series maps were also used to correlate seismic

marker horizons with limestone lithostratigraphy. Two significant limestone formations are

transected by Line SG89-02V: the Gully Oolite and the Friar's Point Limestone. Emphasis is

placed here on the Chadian Gully Oolite of Waters & Lawrence (1987) because the

extrapolation of a prominent marker horizon to surface predicts that it originates within

middle-upper sections of this formation (see details on reflector A in section 5.5).

General estimates of thickness of Carboniferous Limestone from BGS maps vary from about

1km in Porthcawl, 800m in St. Brides to about 600m in Taff's Well.

CHAPTER FIVE


SEISMIC INTERPRETATION, VALE OF GLAMORGAN -8

Upper Carboniferous

Two adjacent areas contain Upper Carboniferous strata: the offshore Inner Bristol Channel,

south of Tusker Rock (BGS 1:250 000 sheet 51°N - 4°W Bristol Channel) and the south crop

of the coalfield. Examination of Namurian isopach maps by George (1970) points to a

maximum thickness of about 500m south of Margam. In line with the general thickness and

grainsize trends for the Namurian a much thinner sequence is probably concealed beneath the

Vale of Glamorgan along an axis from Wick to Penmark and probably consists of finer

sediments than those exposed further north. However the presence of Westphalian rocks

adjacent to Carboniferous Limestone south of Tusker Rock may suggest an absence of

Namurian south of the Cowbridge Anticline.

5.5 THE VALE REFLECTION LINES: DATA DESCRIPTION AND RESULTS.

5.5.1 SECTION SG89 02V, MIGRATED STACK

This section (Fig. 5.4a) reveals four seismic markers (A to D) which represent major

lithostratigraphic divisions and outline the Variscan structure. Reflectors E and F represent

segments of a major Variscan thrust which may substantially repeat the seismic markers.

Reflector A

Seismic reflectors are prominent across the whole section and extend to two-way reflection

times of 2.75s at the southern end of the line (Fig. 5.4b). The section reveals a thin

discontinuous veneer extending down to 0.2s TWTT which discordantly overlies an undulose

prominent reflector (reflector A) in the southern 7.5km of the section (Fig. 5.5a). Reflector A

varies in depth from surface to 0.44s TWTT. The discordant contact is taken to represent the

unconformity between the folded Upper Palaeozoic and the horizontal Mesozoic above.

Two possible interpretations of reflector A are that it represents (1) a significant facies

boundary within the Carboniferous Limestone and (2) the boundary between the Old Red

Sandstone and the Carboniferous Limestone.

In support of (1) above, the up-dip extrapolation of reflector A reaches outcrops of Chadian

Gully Oolite (Fig. 5.5a). Friar's Point Limestone crops out stratigraphically beneath the

upward extrapolation of reflector A. There is evidence for rapid facies changes on Flat Holm

(Fig. 5.5b). Interbeds of indurated limestone and soft shale are expected to be reflective in

seismic experiments. Similar intra-Dinantian facies changes could give rise to reflector A and

package A (refer to section 5.5.2 reflector A).

CHAPTER FIVE



Indirect evidence for (1) above is that the boundary between the Carboniferous Limestone and

Devonian along the south crop is indistinct (Fig. 5.6). However the absence of a decisive

boundary may also be due to a number of geophysical reasons such as interference due to the

moderate dip of Carboniferous Limestone and a low signal to urban noise ratio.

CHAPTER FIVE



In support of (2) above, if a seismic velocity of 5.2kms-1 is assigned to the thickest part of the

sequence (0.25s TWTT) directly above reflector A, the sequence has an actual thickness of

about 700m. This value is in close agreement with 1km thickness estimated for the

Carboniferous Limestone of Porthcawl (George, 1970). The prominence of reflector A

suggests a boundary of major acoustic impedance contrast. A significant contrast is expected

for the Devonian-Carboniferous boundary because seismic velocity changes from 5.2kms-1

downwards to 4.8kms-1.

CHAPTER FIVE



Layer B

There is a transparent zone (layer B) occupying about 0.45s TWTT beneath reflector A (0.45 -

0.9s TWTT at the southern end of the section). Beneath layer B there are prominent reflection

events eg, from 0.9 to 1.55s TWTT beneath shot point 440 (Fig. 5.8) defining an underlying

layer C. Traced northwards up dip, layer B reaches surface in an area of ORS outcrop in the

core of the Cowbridge Anticline. Assigning a seismic velocity of 4.8kms-1 to layer B,

characteristic of the ORS (Mechie & Brooks, 1984), then 0.45s TWTT represents a thickness

of about 1km. This compares well with the thickness estimated from the stratigraphy of the

Cardiff district. Assuming no major stratigraphic omissions between layers B and C the upper

boundary of layer C represents a horizon close to the Devonian-Silurian boundary (see

discussion for possibility of a Siluro-Devonian unconformity).

CHAPTER FIVE



CHAPTER FIVE



Layer C

The prominent reflectors of layer C are inferred to represent lithologically distinctive layers

near the top of the local Silurian sequence eg in the Rumney and Usk inliers. Layer C may

also be partly equivalent to Silurian strata of the Bristol district and the Welsh Borderlands or

SW Dyfed.

CHAPTER FIVE



Folding and discordant relations occur in parts of layer C, for example beneath shot point 600

at 0.9s TWTT (Fig. 5.8). On a kilometre scale, Layer C is folded by the Cowbridge Anticline

and lies closest to the surface along its axial plane at 0.5 to 1.1s TWTT. At the levels given

above there are discordant packages of reflectors. In contrast, the simplest sequence, in layer

C, is to the south of the discordant packages and consists of an upper unit of prominent

reflectors from 0.9-1.2s TWTT above a middle transparent unit from 1.2-1.4s TWTT and a

lower reflective unit from 1.4-1.55s TWTT (Fig. 5.8).

CHAPTER FIVE



Assigning to layer C a seismic velocity of 5.5kms-1, characteristic of Lower Palaeozoic rocks

in the Bristol Channel area (Mechie & Brooks, 1984), 0.65s TWTT (0.9 to 1.55s) represents a

thickness of about 2km. This greatly exceeds the exposed thickness of Silurian in Cardiff,

Bristol or the Welsh Borderlands.

Two possible interpretations for the composition of layer C are: (1) a thick Ordovician-

Silurian sequence analogous to that of SW Dyfed; (2) a thin Silurian sequence of about 400m

above a Cambrian-Silurian unconformity below which there is a thick Cambrian and possibly

Precambrian sequence of about 1600m.

Layer D

Layer D (directly beneath layer C, below shot point 420) has a similar appearance to layer C

consisting of two reflective units bounding a middle transparent zone (Fig. 5.8). Layer D,

extends from 1.75-2.5s TWTT which represents a depth converted thickness of 2km at an

interval velocity of 5.5kms-1. If Layer D represents an indurated layered Precambrian

sequence of higher velocity (eg 6.0kms-1) a depth converted thickness of 2.3km is obtained.

Three possible interpretations for the composition of layer D are (1) part of a complete Lower

Palaeozoic and Precambrian sequence; (2) a tectonic repetition of layer C as suggested by

Hillier (1992); and (3) exclusively Precambrian acid volcanic series of SW Dyfed.

Structure

Reflector A is folded into a gentle synform and shows southward verging minor folds on its

southern limb and downward, northward verging discordant events close to its axis, further

north (Fig. 5.5). The axis of the synform lies beneath the village of Penmark. Closely spaced

sub-prominent reflectors occur beneath reflector A outlining the synform (Fig. 5.5) and the

structure is here named the Penmark Syncline. The discordant events within the reflector

package may represent back thrusts and refolded northward directed thrusts of the type

observed in Carboniferous Limestone quarries and MCM opencast coal sites along the south

crop and in the Gower (Fig. 5.7). The Penmark Syncline is also developed to a lesser extent

within the Mesozoic veneer.

The main structure displayed on the seismic section involves the northern limb of the

Penmark Syncline and the northward dipping south crop of the coalfield which is imaged in

the northern part of the section. From inspection of geological maps of the area (BGS 1:50

000 series sheet 261/262 Bridgend) this is here defined as a section through the Cowbridge

Anticline.

CHAPTER FIVE



The structure observed in layer C beneath the Cowbridge Anticline may represent a complexly

thrusted fold or possibly a decametre scale antiformally stacked thrust duplex (Fig. 5.9). The

strong, moderately southward dipping event (reflector E) shows strong discordance with

weaker reflectors below but is sub-parallel to reflectors above. Reflector E could be correlated

with the sub-horizontal reflector F, to the north, (Fig. 5.9) on the basis of seismic prominence

and tentatively extended southwards between layers C and D.

In this model Reflectors E-F represent a major thrust, repeating the Lower Palaeozoic

sequence beneath the southern limb of the Cowbridge Anticline, which decreases rapidly in

dip below shot point 1060. In this model the Cowbridge Anticline can either be described as

of character like a ramp anticline or even an antiform above a stacked duplex (section 5.7). In

a thrust repeat model for layers C and D, involving a condensed Lower Palaeozoic sequence,

Silurian strata would occupy about 0.2s TWTT so that the remaining 0.45s TWTT would

represent repeated pre-Silurian strata and basement. In the case of a pre-Llandoverian

unconformity model about 1.6km of Cambrian and Precambrian would be repeated once:

� Silurian 2(400m) + Cambrian 2(1.6km) = 4km of layered sequence.

CHAPTER FIVE



A diffuse zone occurs between shot points 1280 and 1400 at depths of 0.7s TWTT, though

some southward dipping reflectors do occur. These contrast strongly with folded prominent

reflectors dipping mainly northwards at the northern limits of the section. The overall view is

of a complex antiform at depth whose northern limb forms a discordance beneath the

northward dipping reflectors of the south crop.

Unlike deeper levels of the section below shot point 460, in the north, below shot point 1360,

there are no distinct reflectors. Below shot points 600 to 860, layer D is gently folded with a

sub-horizontal southern limb and a gently north-dipping northern limb. The hinge of the fold,

at the top of layer D, is 3.5km to the south of the complex antiformal stack above.

5.5.2 SECTION SG89 03V, FILTERED STACK

Section SG89 03V runs approximately along the trend of the Cowbridge Anticline and clearly

displays the Mesozoic veneer, down to 0.1s TWTT, above deeper reflectors at 0.3s and 0.75s

TWTT which probably represent Palaeozoic strata. The unconformable relationship between

the Mesozoic and sub-Mesozoic sequence is excellently displayed beneath shot points 820-

840 where gently dipping sub-Mesozoic reflectors are truncated by the horizontal Mesozoic

reflectors (Fig. 5.10).

Reflector A - package A

The most prominent reflector at shallow levels continues, almost unbroken, across the whole

section (Fig. 5.10). The event is weakest between shot points 880-1080 but strengthens

towards shot point 1320.

CHAPTER FIVE



This major reflector can be correlated directly with reflector A on section SG89-02V and is

thus taken to represent a horizon within the Chadian Gully Oolite or the base of the

Carboniferous Limestone.

Underlying weaker events, such as at 0.4s TWTT beneath shot point 340, represent layers

within the lower part of the Carboniferous Limestone sequence. The entire package of

reflectors (package A) associated with reflector A extends vertically from 0.2-0.45s TWTT,

which represents a thickness of 700m. The complete sequence from the line of the

unconformity to the base of the package A is about 900m thick. This probably represents the

thickness of Carboniferous strata in the southern part of the Vale of Glamorgan. It is

interesting to note the similarity between package A and the South Scarweather package

imaged on offshore seismic data (Chapter 6). Correlation of seismic signature from this line to

dip sections beneath the Bristol Channel could represent a means of establishing a

stratigraphic marker horizon for the offshore data.

Layer B

Beneath package A there is a transparent zone from about 0.45-0.75s TWTT below shot point

720. This is best developed under the section of line SG89 03V between N-S lines SG89 01V

and SG89 02V, but is nevertheless correlated, on the grounds of position and seismic

character, with layer B (Devonian ORS). Layer B thins rapidly towards the west over a

distance of about 5km and is discordant with reflectors below. The seismic nature of the

sequence beneath this layer is very similar to layer C on line SG89 02V. There is the same

threefold constitution with a thick reflective upper layer and a thin, prominent, lower layer

bounding a middle, transparent zone (Fig. 5.11).

CHAPTER FIVE



Layer C

Layer C clearly dips towards the ESE. Picking a horizon within its upper layer, it dips from

0.7s TWTT to 0.85s TWTT over 3km indicating a depth converted apparent dip of about 8°.

Package A and layer B above are sub horizontal and truncate layer C.

There is good evidence for discordances also within layer C, such as at 1.1s TWTT below shot

point 720 (Fig. 5.11).

There are two possible interpretations for the discordances between package A and Layer C

and within layer C (1) structural and (2) stratigraphic. In support of (1) above, layer C may

contain a thrust such as one indicated by reflector E which climbs towards the west of the

strike section possibly up to Carboniferous Limestone levels. In support of (2) above, the dip

of the Lower Palaeozoic sequence can be attributed to late Caledonian structure which caused

the variation in the thickness of the ORS (layer B) above, and an unconformity similar to that

near the Carreg Cennen Disturbance (George, 1970).

CHAPTER FIVE



5.5.3 SECTION SG89 01V, MIGRATED STACK

The southern end of the section contains the most prominent reflectors, at shallow levels 0-

0.35s TWTT. The Mesozoic veneer extends down to 0.08s TWTT and unconformably

overlies a weakly reflective sequence above a strong event correlated with reflector A of

previous sections (Fig. 5.12). There is a gently folded syncline beneath the veneer which can

be traced down to reflector A and probably represents the westward continuation of the

Penmark Syncline (Fig. 5.12). The structure to the north is far less evident than on line SG89

02V. However reflector A can be traced northwards to shot point 1280. A thin south dipping

package, package A2, of closely spaced reflectors extends from 0.2s TWTT below shot point

1240 to the surface at shot point 1100 where Barry Harbour Limestone crops out (Fig. 5.13).

Package A2 and reflector A probably represent horizons within the Carboniferous Limestone.

The contrast in seismic signature between package A2 and reflector A suggests they represent

different horizons so that reflector A cannot be simply extrapolated northwards through

package A2.

CHAPTER FIVE



Below package A2 there are further prominent reflectors which dip towards the north between

shot points 1280-1000. There is a distinct discordance between these two packages (Fig.

5.13). The most prominent lower reflector is here interpreted as a back thrust and has a similar

appearance to discordances imaged on the previous sections.

Assuming the discordances on line SG89 03V are tectonic, a possible interpretation is that the

back thrust rises in structural level from Lower Palaeozoic horizons, in the east, to

Carboniferous Limestone, in the west. The eastward plunge of the postulated thrust line on

section SG89 01V explains why the structure of the Rumney inlier further east is not thrusted.

Thrusting would be below the Silurian.

The lower package of reflectors beneath the thrust is seismically similar to reflectors of layer

C, though layer C on this section is less persistent. The only reflectors in the north are between

0-0.9s TWTT and represent the moderately northward dipping limb of the Cowbridge

Anticline (Fig. 5.14). The syncline imaged at the north end of the section is probably the

Caerphilly Syncline in a seismically-reflective Coal Measure sequence (Fig. 5.15).

There are a number of short discontinuous discordant reflectors beneath the Caerphilly

syncline which may represent the northward continuation of thrusting or the southward

continuation of the Moel Gilau Fault but it is unclear whether it is directly linked with

thrusting evident in the southern part of the section.

CHAPTER FIVE



CHAPTER FIVE



5.6 SUMMARY OF STRATIGRAPHIC MODELS

Seismic reflection template:

Layer TWTT(s)

veneer 0-0.2

base of veneer - reflector A 0.2-0.4

reflector A - top layer B 0.4-0.45

layer B 0.45-0.9

layer C 0.9-1.55

base layer C - top layer D 1.55-1.75

layer D 1.75-2.5

Seismic refraction velocity template (Mechie & Brooks, 1984):

Layer Velocity kms-1

Mesozoic 3.4

Upper Carboniferous 4.8

Carboniferous Limestone 5.2

ORS 4.8

Lower Palaeozoic 5.5

Precambrian 6.0-6.2

Seismic correlation based on sections 5.4 & 5.5:

The veneer represents the Mesozoic; reflector A represents a horizon in Carboniferous

Limestone or the boundary between the Carboniferous Limestone and the Devonian; layer B

represents ORS; layer C represents Lower Palaeozoic; layer D represents a continuation of the

Lower Palaeozoic sequence or a layered Precambrian series or a thrust-repeat of layer C; sub-

layer D represents Precambrian crystalline basement.

Model 1 (reflector A within C. Lmst., U. Carb. preserved):

Layer Thickness (km)

Mesozoic 0.4

Carboniferous 0.7

Model 2 (reflector A within C. Lmst, no U.Carb. preserved):


Mesozoic 0.4


CHAPTER FIVE



Model 3 (reflector A is boundary between C. Lmst. & ORS):


Mesozoic 0.4


ORS 1.1

Model 4 (layer D is Pc layered series):


Mesozoic 0.4


ORS 1.1


Pc layered series 2.5

Model 5 (layer D is repeated layer C):


Mesozoic 0.4


ORS 1.1


Model 6 (discordances between lower layer B and upper layer C and within layer C are

unconformities):


Mesozoic 0.4


ORS 1.1 (maximum)

Silurian 0.4

Pre-Silurian 3.5

Depth Model A (based on models 1-4):

Boundary Depth (km)

Base of Mesozoic 0.4

Carboniferous Limestone-ORS 1.0

ORS-Lower Palaeozoic 2.0-2.5

Lower Palaeozoic-Pc layered series 4.0

Pc layered series-Pc basement 6.5

CHAPTER FIVE



Depth Model B (based on model 5):

Boundary Depth (km)



ORS-Lower Palaeozoic 2.0-2.5

Lower Palaeozoic-Pc basement 6.5

Depth Model C (based on model 6):

Boundary Depth (km)



ORS-Silurian unconformity 2.0

Silurian-Cambrian? unconformity 2.5

Lower Palaeozoic-Precambrian basement 6.5

unconformity

5.7 DISCUSSION ON THE STRUCTURE

The descriptions of the Vale seismic lines show that structural complexity is concentrated at

sub-Devonian levels. The Cowbridge Anticline at surface is associated at moderate depth

(1.5km) with a localised structural complexity and, at about 5km depth, with a possible

basement related antiform. It is possible that major thrusts involved in the structural

complexity repeat the supra-basement sequences described in section 5.6 (Fig. 5.16; eg

reflectors E, F) and that another major thrust deforms layer D below. Thrusts mapped in the

Vale of Glamorgan (BGS 1:50 000 series sheet 261/262 Bridgend), however, have decametre

scale displacements. Seismically imaged thrusts beneath the Cowbridge Anticline are

correlated with these exposed thrusts and are interpreted to be part of a Variscan imbricate

fan. The cumulative displacement is questionable.

The general structure of the Cowbridge Anticline is of a northward facing fold with a broad

crest 2 to 3km wide. The localised prominent discordant reflectors which either represent a

complex thrust-related fold or duplex occur immediately to the south of the axis of the

Cowbridge Anticline. The best analogies of duplexing of this type were discovered in the

Carboniferous Limestone near Rhossili and MCM of Gilfach Iago (Chapter 4, Figs 4.36 &

4.50). Mesoscale structure was used as an analogue for restoring the decametre-kilometre

width duplex beneath the Cowbridge Anticline to obtain an estimate of shortening and

associated uplift of the south crop.

CHAPTER FIVE



An estimate of uplift is justifiable in the case of the duplex being linked to thrusts further

north (Fig. 5.17) eg back-thrusting and antiformal stacking in Taff's Well Quarry (Fig. 5.18).

Antiformal stacking can cause back-thrusting at higher stratigraphic levels due to associated

underthrusting. Models of underthrusting have been applied to the Apennines of N Italy and

have been described by Vann et al (1986). Jones (1991) has applied Vann's model of

mountain front geometry to the structure beneath the Vale of Glamorgan so that the details of

the model are now in question and clearly can been analysed by two means: (1) structural

description of thrusting in the Vale of Glamorgan; and (2) direct examination of the seismic

sections.

There are clear problems in applying the above model of mountain front geometry to the Vale

structure:

CHAPTER FIVE



CHAPTER FIVE



1. The postulated mountain front structure appears to be restricted to the Vale even though the

passive-roof duplex geometry in the frontal structures of the Kirthar and Sulaiman mountains

in Pakistan is at least 70km long.

2. There is no direct evidence for major back thrusting at Carboniferous Limestone or

Devonian levels other than thrusts of mesoscale displacement.

3. There is no evidence for the along-strike occurrence of the major duplex beneath the Vale

predicted to be the cause of the uplift of the south crop. Rather, outcrops of Silurian strata

provide evidence for a simple stratigraphic contact with Devonian strata.

The problems above are investigated:

1. Why should such a model be accepted for the Vale when the main evidence for large-scale

back thrusting, underthrusting and stacking occurs in North Cornwall and Devon south of the

Rusey Fault? Furthermore, assuming a similar scale of mountain front structure to that

described by Banks & Warburton (1986) it would be expected that the duplex would extend

southwards into the Bristol Channel. In fact there is no evidence for a series of large scale

discordant reflector packages in either the Vale or the Inner Bristol Channel. Instead there is

evidence for singular thrusts of major length but uncertain displacement (Chapter 6).

However in argument for major structural shortening in the Vale would be that shortening

observed within the coalfield of about 40% should be related to a major structure in the

hinterland at depth. i.e. all the mesoscale structures observed in the OCCS are related to

thrusting in the Bristol Channel and the Vale, in the case of a linked system. As a general

estimate the South Wales coalfield of present width of about 30km would have been

shortened by 12km if a shortening of 40% is taken. Therefore the root structure to the south of

the coalfield beneath the Vale should possess a displacement of at least 12km. This is indirect

evidence for a major thrust beneath the Vale dramatically repeating a condensed sequence.

In contrast to a simple linked system would be a structural system involving basement

structures around which shortening was concentrated in the foreland to give anomalously high

values of shortening. If this argument is correct then the structure observed in the OCCS could

represent local strain and consequently overemphasise shortening estimates. It would then be

reasonable to assume a much smaller displacement on structures beneath the Vale. However

the structure observed in the OCCS usually contrasts with the structure of basement related

faults in SW Dyfed (Powell, 1987). The argument above is ill-founded.

CHAPTER FIVE



2. The surface geology and the seismic structure beneath the Vale of Glamorgan differ from

the model section of Jones (1991). It is not conclusive however whether a thrust-repeated sub-

Devonian sequence uplifted the south crop in a similar style to other mountain front models of

Vann et al (1986). The alternative is that the sub-Devonian sequence is not repeated and that

the small zone of complexity represents a locally-formed stack near the point at which the

associated thrust changes from a ramp to a flat, in which case the Cowbridge Anticline is a

ramp anticline of similar geometry to that described by Suppe (1983) and Jamison (1987)

above a thrust of limited displacement.

Figs. 5.20 a, b & c and Enclosure 5 consist of sections representing various models of the

structure beneath the Vale of Glamorgan (imbricate fan versus duplex; thrusts with small

displacements versus thrusts with large displacements). The displacement along the thrust

represented by reflector E may be about 10km (based on the correlation of layers C & D)

whereas if general shortening of the Carboniferous Limestone across the Cowbridge Anticline

is considered a minimum value of about 1-2km is obtained. This does not however take

account of small-scale back thrusting along the south crop, eg at Taff's Well.

3. Geological surveys were carried out in areas of the Vale of Glamorgan in which Lower

Palaeozoic strata crop out and also to the north-west of the coalfield in the Ordovician and

Silurian inliers of the Welsh Borderlands. The Lower Palaeozoic strata of the Welsh

Borderlands show a moderate dip probably due to movement along the Carreg Cennen Fault

and are a good structural standard for comparison with Lower Palaeozoic strata of the Vale.

The Silurian inlier in Cardiff, consisting of Ludlovian Rumney Grits, was examined to

observe the local nature of deformation in Lower Palaeozoic strata, in order to test the model

of major thrusting. Unlike the locally complex structure observed on the seismic sections, the

Rumney Grits merely show a regional dip due to Variscan folding. Associated bed-parallel

slip with slickenside lineations plunging down dip and minor faulting were also observed.

Two possible explanations for the absence of major structures at Siluro-Devonian levels in

Cardiff are:

(a) north-west trending faults such as the Variscan root-structures of the Cothelstone Fault

(Chapter 6) and the St. Mary's Well Bay Fault-Dinas Fault (Chapter 7), compartmentalise the

Variscan deformation of the Vale of Glamorgan from the offshore structure to the west and to

the east in the Bristol district. The north-west trending Variscan fault such as in Ruthin

Carboniferous Limestone Quarry is a good analogue of a north-west lateral or oblique ramp-

thrust subdividing the Variscan structure (Fig. 5.19).

CHAPTER FIVE



(b) The scale of duplexing beneath the Vale is too small to reach even the Cardiff district to

the east.

The shallower parts of the Vale reflection lines show prominent reflectors (including reflector

A) probably representing horizons within the Carboniferous Limestone or the boundary

between the Carboniferous Limestone and the ORS. The Penmark Syncline, in the south,

contrasts with the observed structure beneath the Bristol Channel which is a southward

dipping event described as limestone south of the Gower (Brooks et al, 1988). If the

southward dipping panel beneath the Inner Bristol Channel and the southern limb of the

Penmark Syncline are taken to represent limbs of the same structure, an anticline must trend

approximately along the coast of the Vale of Glamorgan. However the Vale and Bristol

Channel reflection lines are on opposite sides of the offshore continuation of the Cothelstone

Fault, rendering uncertain the correlation of the onshore and offshore events (Chapter 6).

It is possible, for example, that the structure of the Vale of Glamorgan cannot be correlated

with the structure beneath the Bristol Channel due to structural compartmentalisation.

Nevertheless field mapping in the Barry area reveals a major anticline close to the Vale coast

and therefore supports the above interpretation of the seismic data.

In summary the general structure of the Carboniferous Limestone is far simpler than the deep

structure.

CHAPTER FIVE



A.

B.

In the absence of a deep borehole the interpretation of the Vale reflection lines remains

uncertain. However various structural and stratigraphic models can be applied to the sections

and supported by analogy with the geology of surrounding areas, such as SW Dyfed, the

Welsh Borderlands and the Mendips.

CHAPTER FIVE



The main uncertainty of the geological interpretation is whether the seismic layered sequence

to reflection times of 2.5s is attributable to a thick stratigraphic sequence or to tectonic

duplication of a thinner sequence. A compromise must therefore be made between structural

and stratigraphic complexity, in line with the regional geology.

C.

The systematic description of the seismic sections clearly emphasises the occurrence of major

discordances at various levels. It is interesting to speculate whether these discordances

represent Variscan thrusts or unconformities associated with pre-Variscan tectonism. It is

equally possible that the high level angular discordances observed on line SG89-03V (Fig.

5.21) are due to Variscan thrusting or to regional sub-Devonian unconformities.

CHAPTER FIVE



In support of a thrust model, there is good surface evidence for thrusting such as in Ruthin

Quarry, Taff's Well Quarry and Trwyn y Witch, but only of decametre scale displacement. The

discordances (postulated to be regional unconformities) are difficult to trace across the

sections. Instead a sub-Devonian sequence about 4km in thickness is imaged. How can all the

discordances represent unconformities in a condensed sequence when there is 4km of layered

sequence? This is a crucial question in the interpretation of the sections since to assign a

stratigraphic identity to a particular discordance in a general case requiring a substantial

amount of thrust repetition is here considered inconsistent.

A thrust-repeated unconformable succession would seem an unreasonably complex

interpretation until the Mendips are examined in which Cambrian and unconformable Silurian

strata are involved in Variscan thrust-related folding. Furthermore examination of Lower

Palaeozoic strata in the Malvern Hills point to their association with Pre-Llandoverian

structure in addition to later Variscan deformation (Brooks, 1970).

CHAPTER FIVE



Thus on the basis of the regional structure to the east of the Bristol Channel Borderlands it is

not unlikely that a complex condensed stratigraphic template coexists with complex

Caledonian and Variscan structures to form the observed thickness of discordantly layered

sequence.

Against a thrust model, in addition to the scale of thrust displacement is that the depths to the

angular discordance occur in reasonable agreement with a depth to major sub-Devonian and

pre-Silurian unconformities predicted from the local thickness of Carboniferous Limestone,

Old Red Sandstone and Silurian. Conversely if a complete sequence such as that in SW Dyfed

occurs under the Vale there is no necessity for major thrusting or major unconformities.

However there are substantial palaeogeographic consequences of such a thick sequence

beneath the Vale (in particular to the south-east of the Carreg Cennen Disturbance). Evidence

from Lower Palaeozoic outcrop to the east, suggests that during the Early Palaeozoic the Vale

of Glamorgan would probably have been situated on a stable continental shelf. The Welsh

Basin on the other hand would probably have undergone fault controlled subsidence to

accommodate the thick sedimentary pile.

It is interesting to note that in either the thrust model or the complete sequence model, the

lower part of the seismic sequence may be interpreted as layered Precambrian, similar to

volcanic and volcaniclastic sequences of SW Dyfed. Also in SW Dyfed there are angular

discordances in upper levels of the section attributable to variations in the thickness of the

Devonian such as those which occur across the Benton Fault.

Further examination of Devonian strata in the Llanstephan area has shown that in addition to

Variscan tectonism (this study), Late Caledonian or Acadian deformation has affected the

region (Cope, 1979). However the seismic sections show no evidence for intra-Devonian

discordances and place any discordance below or at the base of the Devonian, based on local

thickness estimates. It is unclear as to whether Caledonian structures developed at depth to

the south of the Carreg-Cennen Disturbance. Evidence presented in Chapter 4 points to

Variscan reactivation of the Caledonoid fault and also to unrelated Variscan thrusting. This

suggests that any structural elements on the Vale reflection lines are of Variscan age. The

most likely explanation for the nature of the Vale reflection lines is that a thin Lower

Palaeozoic sequence similar to that of the Welsh Borderlands overlies a Precambrian

sequence similar to that of SW Dyfed, and these sequences are involved in Variscan thrusting

as observed, eg on Gower and in the Mendips. If there is a major repetition of the sub-

Devonian sequence beneath the Vale of Glamorgan, then associated major back thrusting and

uplift of the Carboniferous Limestone of the south crop are required as suggested by Jones

(1991).

CHAPTER FIVE



However, if thrust displacements at depth are kept to a minimum, section balancing reveals

the greatest conformity in structural style vertically through the sections, so that Variscan

thrusting of substantially less than 1km may characterise the Vale of Glamorgan contrary to

previous models involving major thrusting.

5.8 UPPER PALAEOZOIC SUBCROP MAPS FOR THE VALE OF GLAMORGAN

AND THE INNER BRISTOL CHANNEL

Consideration of the sub-Mesozoic stratigraphy beneath the Vale of Glamorgan clearly sheds

light on the deep structure of the Bristol Channel. The occurrence of Lower Palaeozoic strata

at shallow depth points to the possible significant control of Caledonian and Variscan folding

in the preservation of offshore stratigraphy. However it is likely that Upper Carboniferous is

preserved offshore assuming that the South Scarweather package (Chapter 6) and the

reflectors on the Vale reflection lines are correctly interpreted stratigraphically. Different

possibilities are present to link the offshore geology of the Inner Bristol Channel with that of

the Vale of Glamorgan, namely, down warping due to a southward facing anticline,

extensional faulting and thrusting.

A south facing anticline may bring Carboniferous Limestone below its surface position in the

Vale of Glamorgan to a depth of 1.5km beneath the Bristol Channel, preserving a sequence of

Upper Carboniferous. Such a structure would be analogous to the structure immediately south

of Tusker Rock near Ogmore.

An extensional fault may downthrow the Carboniferous Limestone achieving the same result

as the former case.

Thrusting and folding may have uplifted pre-Mesozoic strata in the Bristol Channel so that

present reflectors represent pre- Carboniferous horizons, the overall structure being analogous

to that of the Mendips.

5.9 CONCLUSIONS

The Vale seismic lines reveal structural information from Precambrian basement level up to

Carboniferous.

Previous estimates of the thickness of the suprabasement Precambrian/Lower Palaeozoic

succession are questionable.

CHAPTER FIVE



Evidence has been found for a Variscan imbricate fan associated with the Cowbridge

Anticline. Small scale thrusting in the Vale of Glamorgan may continue at depth and probably

caused the uplift of the south crop. The structural model above is an alternative to the large

scale mountain front duplex of Jones (1991).

The inferred scale of displacement on the seismically imaged thrusts is dependant on the

stratigraphic model used in seismic interpretation.

Back thrusting along the south crop could be related to limited underthrusting, above a

basement structure, expected about 50km north of the major mountain front duplex of North

Cornwall.

Correlation with the seismic sequence in the Bristol Channel suggests the presence of Upper

Carboniferous strata beneath the Inner Bristol Channel.

Appendix 5.1

Appendix 5.2

GWBASIC and FORTRAN software used for depth conversion of seismic data from the Vale

of Glamorgan and the Bristol Channel.

program digit

c FORTRAN program to calculate

c the depth of reflectors from

c Vint and T2 data

CHAPTER FIVE



real v(1:30)

real t(1:30)

real w(1:30)

real d(1:20, 1:50)

real s(1:30)

integer it2, ire

t(0)=0.

write(*,500)

print*,'A program to convert T.W.T.T. into real depths,',

& 'from Interval Velocities.'

c Input Section

write(*,500)

write(*,1000)

read(*,*)it2

write(*,500)

write(*,1500)

write(*,2000)

500 format (///)

1000 format ('Enter the number of T.W.T.T. values.'/)

1500 format ('Enter values of Interval Velocity and T.W.T.T.')

2000 format ('in ascending order.'/)

3000 format (10x,'Vint (km/s)', 12x,i,/10x,$)

3500 format (10x,'T.W.T.T. (s)',12x,i,/,10x,$)

6000 format ('True depths of intersection of the reflectors on,'

& 'the C.D.P. Axis are:')

7000 format (10x,'Reflector', 25x 'Depth (km)'/)

8000 format (12x, i3, 30x, f6.3)

8500 format ('Enter the number of reflectors which cross cut the,',

& 'C.D.P. Axis.'/)

9000 format (10x, 'Reflector ',12x,i,/)

do 10 ix=1, it2

write(*,3000) jx

read(*,*) v(jx)

write(*,3500) jx

read(*,*) t(jx)

continue

write(*,500)

write(*,8500)

CHAPTER FIVE



read(*,*) ire

write(*,500)

do 20 ix=1,ire

write(*,9000) jx

write(*,3500)

read(*,*) w(jx)

print*

20 continue

c Calculation Section

do 30 ix=1,ire

do 40 jy=1,it2

if (w(jx).ge.t(jy) then

d(jx,jy)=((t(jy)-t(jy-1))/2.)*v(jv)

else

d(jx,jy)=((w(jx)-t(jy-1)/2.)*v(jy)

goto 30

endif

40 continue

30 continue

do 50 jx=1,ire

s(jx)=0.

50 continue

do 60 jx=1,ire

do 70 jy=1,it2

s(jx)=s(jx)+d(jx,jv)

70 continue

60 continue

c List Results Section

write(*,500)

write(*,6000)

write(*,500)

write(*,7000)

do 90 jx=1,ire

write (*,8000) jx,s(jx)

90 continue

write(*,500)

stop

end

CHAPTER FIVE



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CHAPTER FIVE



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CHAPTER FIVE



FIGURE CAPTIONS

Fig. 5.1 Location map for the Vale seismic reflection sections. Key: Sw Swansea, PT Port Talbot, Pcl Porthcawl, B

Bridgend, Cf Cardiff, Po Pontypridd, Mae Maesteg, GG Gilfach Goch, Cy Caerphilly, Ma Machen, Pe Pencoed, Ll

Llantrisant, Mi Miskin, W Wick, StA St Athan, Pk Penmark, StD St Donats, LlM Llantwit Major, Rh Rhoose, Og Ogmore.

Main roads: M4, A48. Section lines: 01V, 02V & 03V.

Fig. 5.2 Borehole data of Cambrian Exploration Ltd. Key: SB, Senghenydd Borehole; MB, Maesteg Borehole; orv,

Ordovician; S, Silurian; ORS, Old Red Sandstone; CL, Carboniferous Limestone. (Depths are in Metres).

Fig. 5.3 a. Geological section through SW Dyfed showing the depth to the major refracting boundary postulated to

represent the surface of the Precambrian crystalline basement (Mechie & Brooks, 1984). Key: JT, Johnston Thrust; numerical

values represent seismic refraction velocities in kms-1.

b. Field sketch looking westwards from near Ticklas at Precambrian crystalline basement overlying Silurian and

Westphalian rocks. The synoptic sketch schematically represents Precambrian rocks thrust over Palaeozoic rocks.

Fig. 5.4 a. General seismic structure beneath the eastern part of the Vale of Glamorgan.

b. Sub horizontal seismic reflectors on line SG89 02V occurring from 0s to 2.75s TWTT.

Fig. 5.5 a. Southern end of section SG89 02V showing a planar veneer extending to 0.2s TWTT above the folded,

prominent reflector (A). The extrapolation of Reflector A to surface is correlated with Carboniferous Limestone inliers of the

Vale of Glamorgan. Minor structures within reflector A probably represent folding and thrusting. The composite nature of

the reflector A 'horizon’ is evident.

b. Lithological contrasts in Carboniferous Limestone on Flat Holm in the Inner Bristol Channel.

Fig. 5.6 Indistinct base to the reflective package representing the Carboniferous Limestone of the South Crop on section

SG89 02V.

Fig. 5.7 a. Location map of sites containing reoriented structures analogous to those shown by reflector A.

b. An example of a reoriented thrust in Carboniferous Limestone of Rhossili on the Gower.

Fig. 5.8 The transparent zone, layer B, beneath reflector A probably representing Old Red Sandstone. The layered sequence

(layer C) below, probably represents Lower Palaeozoic strata. The boundary between B and C could represent the Devono-

Silurian boundary, as it crops out in the Cardiff District. Angular discordances occur in layer C in middle parts of section

SG89 02V. The figure also shows the composite, tripartite layering of layers C and D. A possible repeated sequence on

section SG89 02V.

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Fig. 5.9 Complex antiformally stacked thrust duplex or thrusted fold above a major change in thrust angle from the south

dipping reflector E to sub horizontal reflector F, on section SG89 02V. The figure shows the correlation of reflectors E and F

as a single thrust line of changing plunge. The prominent reflector beneath E may represent another thrust flat.

Fig. 5.10 Strike section SG89 03V revealing the structure of reflector A and package A, correlated with a horizon within the

Carboniferous Limestone or a horizon close to the Devono-Carboniferous boundary. Part of strike section SG89 03V shows

the truncation of Palaeozoic reflectors by the Mesozoic veneer at about 0.1s TWTT.

Fig. 5.11 The tripartite structure of layer C on section SG89 03V.

The discordance within layer C represents either a strike section through a thrust or an unconformity at middle levels of

section SG89 03V.

Fig. 5.12 The westward continuation of the Penmark Syncline is outlined by reflector A beneath the Mesozoic veneer of

section SG89 01V.

Fig. 5.13 A thin package of closely spaced reflectors intersect outcrop of Barry Harbour Limestone on section SG89 01V.

There is a clear discordance in reflectors beneath shot points 1230-1080. The discordant reflector packages possibly represent

bedding and a ramp thrust.

Fig. 5.14 North dipping limb of the Cowbridge Anticline on section SG89 01V.

Fig. 5.15 Seismically reflective Coal Measure sequence of the Caerphilly Syncline at the north end of section SG89 01V.

Fig. 5.16 A possible repetition of the suprabasement sequence by a thrust extended southwards from reflector E. Another

thrust deforms layer D in the footwall of the thrust above.

Fig. 5.17 A hypothetical linked thrust system incorporating the structure beneath the Cowbridge Anticline and the south crop

of the coalfield. Thrusting beneath the Vale could be related directly to the uplift of the south crop.

Fig. 5.18 An antiform folding thrusts (E) and other south and north verging thrusts (A-D) in Carboniferous Limestone of

Taff's Well quarry situated on the south crop of the coalfield north-west of Cardiff.

Fig. 5.19 North-west trending lateral Variscan fault in Carboniferous Limestone of Ruthin Quarry. A possible analogue of a

compartmentalising cross fault.

Fig. 5.20 a, b & c Structural sections based on the Vale reflection lines.

Fig. 5.21 Angular discordances on line SG89 03V representing either thrusts or sub-Devonian unconformities.

Marios Miliorizos 12th June 2006 File name: PhD Chapter 5 Five

tectonic evolution of the bristol channel borderlands chapter 5

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