tectonic evolution of the bristol channel borderlands chapter 0 introduction

INTRODUCTION Tectonic Evolution of the Bristol Channel borderlands

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INTRODUCTION

I. LOCATION

The Bristol Channel borderlands are shown in Fig. I.1. They are divided into the following areas for

convenience of study: SW Dyfed, South Wales coalfield, Llanstephan-Carreg Cennen, Vale of

Glamorgan, inner Bristol Channel, Somerset, north Devon, north Cornwall. Three main areas of

Upper Palaeozoic geology are north Devon, Cornwall and South Wales. Areas of local Lower to

Upper Palaeozoic interest are the Llanstephan-Carreg Cennen area and SW Dyfed. The inner Bristol

Channel and the Vale of Glamorgan are areas of seismic reflection study. Areas of Mesozoic

interest include the Vale of Glamorgan and Somerset. Surrounding areas such as south Devon, the

Bristol district and the Welsh borderlands were investigated by review of literature relevant to areas

within the Bristol Channel borderlands.

BRISTOL CHANNEL BORDERLANDS

Figure I.1 The Bristol Channel borderlands, subdivided into areas for convenience of study.

Key: P Pembrokeshire (SW Dyfed); CC Carreg Cennen-Llanstephan; SWCB South Wales coal

basin; G Gower; V of G Vale of Glamorgan; M Monmouth; B Bristol district; S Somerset; ND

north Devon; NC north Cornwall; SD&SC south Devon & south Cornwall; and Bristol Channel.

II. ROCK TERMINOLOGY

The sandstone classification of Gilbert (in Williams et al. 1954) and the carbonate classification of

Dunham (1962) have been adopted for the thesis.


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III. GENERAL STRATIGRAPHY

Precambrian to Tertiary strata are preserved within the Bristol Channel borderlands. The general

thickness of sedimentary sequence above crystalline basement is about 6km. There is about 2000m

of Precambrian exposed in SW Dyfed (Baker, 1982); 4000m of Lower Palaeozoic in SW Dyfed

(George, 1970) a minimum of 350m in Monmouth (BGS 1:63,360 series, sheet 233) and over 600m

in the Welsh borderlands (Brooks, 1970). There are 5300m of Upper Palaeozoic in Devon

(Durrance & Laming, 1982; BGS 1:50,000 series, sheets 292 & 307/308, Bideford and Lundy and

Bude) and 5500m maximum in South Wales (George, 1970). There are up to 400m of Mesozoic in

the Vale of Glamorgan and about 1500m beneath the inner Bristol Channel (from seismic data).

There are local sequences of Tertiary in isolated basins in Devon up to 800m in thickness (Freshney

et al. 1979).

The study region contains major sedimentary basins. The Mesozoic Bristol Channel basin is

superimposed on Late Palaeozoic basins along the northern outer arc of the Variscan orogenic belt:

the Devonian north Devon basin, the Culm thrust sheet top basin (Gayer & Jones, 1989) and the

South Wales coalfield foreland basin (Kelling, 1988). To the north of the borderlands are the Welsh

basin and Devonian molasse sediments of the Brecon Beacons whilst to the east are the Mesozoic

and Tertiary strata of the Wessex basin flanking the Upper Palaeozoic inliers of the Mendip and

Quantock hills.

The Upper Palaeozoic stratigraphy of the Bristol Channel borderlands contains several major

unconformities, eg, between the Lower Devonian and Upper Devonian beneath the Vale of

Glamorgan and the Carboniferous Limestone and the Millstone Grit along the east crop. The most

pronounced regional unconformity occurs between the pre- and post-Variscan Groups and yet it

represents a short period of time, partly Stephanian and Permian times. Another significant

unconformity occurs between the Upper Jurassic and Upper Cretaceous of the Bristol Channel

basin. These unconformities mark periods of significant basin inversion. Fig. I.2 summarises the

general stratigraphy of the Bristol Channel borderlands. The reader is referred to Chapter 2 for the

broader setting of the study area within the western European Variscan orogen.

IV. AIMS

Having begun a detailed study of exposures in north Devon and South Wales it became clear that

the regional reactivation events identified by e.g. Powell (1987) and Le Gall (1991) could be

subdivided into discrete episodes.


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This is attempted for the Variscan structure. The style of deformation in north Devon due to

composite fault movement could be compared with the models of progressive deformation

proposed by the Cardiff Coalfield Research Team for the South Wales coalfield.

Figure I.2 General stratigraphy of the Bristol Channel borderlands. Composite stratigraphic column

from George (1970). Key: Pc xtal Precambrian crystalline basement; Pc layered Precambrian

layered sequence; LPlzc Lower Palaeozoic sequence; Trmc Tremadocian strata; sln Silurian strata;

UPzc Upper Palaeozoic, mzc Mesozoic, Try Tertiary sequences.

A major aim of the project was to define the concealed Variscan structure of the Bristol Channel

and the Vale of Glamorgan using seismic reflection data. In the process of describing the Variscan

structure of the Bristol Channel borderlands a methodology was devised where onshore stratigraphy

and structure were used as analogues to assist seismic interpretation.

Another aim of the project was to identify areas of Mesozoic and Tertiary geology which illustrate

the reactivation of Variscan faults. This was a good means of completing the study because it would

involve the remainder of the tectonic evolution of the Bristol Channel borderlands.

The detailed aim of Chapters 3 & 4 is to present the geological history which describes further

reactivation events to those described in the crustal evolutionary model, for South Wales and north

Devon, proposed by Le Gall (1991). Powell (1987) and Le Gall (1991) emphasised the structural

evolution from a Devonian basement framework to the final Variscan structure. However mesoscale

details across the Bristol Channel borderlands have previously been overlooked.

The geology of the Bristol Channel borderlands is subdivided by the Bristol Channel fault zone,

linked to underlying Variscan thrusts (Brooks et al. 1988, Miliorizos, 1991). The contrast in style of

deformation between the hangingwall and footwall blocks of the Bristol Channel fault zone (BCFZ)


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has been highlighted by Gardiner & Sheridan (1981) and Gayer & Jones (1989). Two lines of

investigation were followed: (1) a comparison was made of the kinematic history and style of

deformation in the hangingwall and footwall blocks; and (2) investigations were carried out to

determine how such contrasting terranes were juxtaposed. Field studies revealed new details of the

mesoscale structure in the Bristol Channel borderlands and complications in the geometry and

number of Variscan thrusts beneath the inner Bristol Channel. Chapter 3 describes the structure of

the hangingwall of the BCFZ; Chapter 4 describes the footwall of the BCFZ. A comparison of the

two blocks illustrates the evolution of the basement related fault-controlled basins in a

compressional foreland setting. Devonian strata in the footwall contrast with the emergent basement

related thrust zone of SW Dyfed and the thin skinned Variscan deformation in the western part of

the South Wales Coalfield. This subdivision of the footwall block is tested in the case of the

reactivation of various basement related faults beneath and within the coalfield.

V. METHODS AND DATABASE

A large structural database has been constructed from (1) coastal sections; (2) observations in

British Coal opencast coal sites; (3) seismic reflection data from MERLIN GECO-PRAKLA and

SHELL UK EXPRO; and (4) other sources such as BGS 1:50,000 series maps and borehole core

from Keyworth, Nottingham, e.g. the Beacons Down borehole, near Ogmore.

The method of research changed with the type of data utilised and with experience. Initial field

work was general in scope and concentrated on description of mesoscale structures. However a

focused view to fieldwork was required to achieve even the initial aims of the project. Fieldwork

progressed to model testing by 1:10,000 scale mapping and stratigraphic surveying. Focused model

testing was especially significant in the analysis of the seismic reflection data. The thesis is thus

essentially descriptive but each chapter is constructed so that emphasis is also placed on modelling

in the research. Details of the seismic interpretation methods are given in the relevant chapters;

most withstand the recent critique by Brooks (1992) on the general approach to seismic

interpretation.

VI. GEOMORPHOLOGY

No attempt is made to describe the geomorphology of the Bristol Channel borderlands. However an

important point to note is that the coastal geomorphology reflects the Variscan and Mesozoic

faulted grain of the country rock. The orientation and structure of many coastal sections are related

directly to bedding, cleavage, fold axial planes and faults. One such example is the shape of coast

near Lynmouth. Another example may be the west coast of the Vale of Glamorgan controlled by

Mesozoic faulting associated with the continuation of the Merthyr Mawr and Cothelstone faults.


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In general the shape of the whole of the inner Bristol Channel is controlled by Recent erosion along

ancient fault lines.

Platform at Southerndown

Cliff folds at Watchet

Waves at Trwyn y Wrach

M. Miliorizos 1988-1992 PhD research.


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VII. REFERENCES

Brooks, M., 1970. Pre-Llandovery tectonism and the Malvern structure. Proceedings of the

Geologists’ Association, Volume 81, part 2, pp. 249-268.

Brooks, M., Trayner, P.M. & Trimble, T.J., 1988. Mesozoic reactivation of Variscan thrusting in

the Bristol Channel area, UK. Journal of the Geological Society, London, Volume 145, pp. 439-

444.

Brooks, M., 1992. Discussion on the crustal evolutionary model for the Variscides of Ireland and

Wales from SWAT seismic data; reply by B. Le Gall with collaboration of C. Bois and the ECORS

SWAT Group. Journal of the Geological Society, London, Volume 149, part 4, p. 681.

Baker, J.W., 1982. The Precambrian of south-west Dyfed. In: Geological Excursions in Dyfed,

south-west Wales, (editor) M.G. Bassett. Published for the Geologists’ Association, South Wales

Group by the National Museum of Wales, Cardiff.

Dunham, R.J., 1962. Classification of carbonate rocks according to depositional texture. In: W.E.

Ham (editor), Classification of Carbonate Rocks. American Association of Petroleum Geologists

Memoir, 1, pp. 108-121.

Durrance, E.M. & Laming, D.J.C., 1982. (Editors) The Geology of Devon, University of Exeter.

Freshney, E.C., Edmonds, E.A., Taylor, R.T. & Williams, B.J., 1979. Geology of the country

around Bude and Bradworthy. Memoirs of the Geological Survey of Great Britain, (sheet 322 N.S.).

Gardiner, P.R. & Sheridan, D.J.R., 1981. Tectonic framework of the Celtic Sea and adjacent areas

with special reference to the location of the Variscan front. Journal of Structural Geology, Volume

3, number 3, pp. 317-331.

Gayer, R.A. & Jones, J., 1989. The Variscan foreland in South Wales. Proceedings of the Ussher

Society, Volume 9, pp. 177-179.

George, T.N., 1970, South Wales, 3rd edition, British Regional Geology, HMSO.

Kelling, G., 1988. Silesian sedimentation and tectonics in the South Wales basin: a brief review. In:

Besly, B. & Kelling, G. (editors), Sedimentation in a synorogenic basin complex: the Upper

Carboniferous of north-west Europe. Publishers: Blackie, Glasgow and London, pp. 38-42.


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Le Gall, B., 1991. Crustal evolutionary model for the Variscides of Ireland and Wales from SWAT

seismic data, Journal of the Geological Society, London, Volume 148, pp. 759-774.

Miliorizos, M., 1991. Sub-Mesozoic stratigraphy and Variscan structure under the inner Bristol

Channel, (abstract). Proceedings of the Ussher Society, Volume 7, part 4, p. 430.

Powell, C.M., 1987. Inversion tectonics in SW Dyfed. Proceedings of the Geologists’ Association,

Volume 98, pp. 193-203.

Williams, H., Turner, F.J. & Gilbert, C.M., 1954. Petrography. Publishers: San Francisco, Freeman.

VIII. FIGURE CAPTIONS

Fig. I.1 The Bristol Channel borderlands subdivided into areas for convenience of study. Key: P

Pembrokeshire (SW Dyfed); CC Carreg Cennen-Llanstephan; SWCB South Wales coal basin; G

Gower; V of G Vale of Glamorgan; M Monmouth; B Bristol district; S Somerset; ND north Devon;

NC north Cornwall; SD & SC south Devon and south Cornwall.

Fig. I.2 General stratigraphy of the Bristol Channel borderlands. Composite stratigraphic column

from George (1970). Key: Pc xtal Precambrian crystalline basement; Pc layered Precambrian

layered sequence; LPzc Lower Palaeozoic; Trmc Tremadocian; sln Silurian; UPzc Upper

Palaeozoic; mzc Mesozoic; Try Tertiary.

M. Miliorizos

30th April 2007

File name PhD Introduction

pages 1 to 7


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tectonic evolution of the bristol channel borderlands chapter 0 introduction

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