tectonic evolution of the bristol channel borderlands chapter 2

CHAPTER TWO

Tectonic Evolution of the Bristol Channel borderlands

FORELAND BASINS

Page 2-1

2. FORELAND BASINS

2.1 INTRODUCTION

The tectonic history of South Wales and North Devon is related to the evolution of peripheral

foreland basins along the northern margin of the Variscan Orogen. The history of a peripheral

foreland basin can be subdivided in to three broad and diachronous stages: sedimentation,

diagenesis and structural deformation. This chapter will concentrate on the tectonic aspects

controlling sedimentation and structural deformation, including fault reactivation.

Before the name foreland basin was proposed, basins lying in front of an orogenic arc were

defined in terms of their polarity as Miogeosynclines and Eugeosynclines (Aubouin, 1965).

Dickinson (1974) introduced the term foreland basin for the polarised eugeosynclines and

miogeosynclines which developed within the European Alpine orogenic system (Fig. 2.1).

Following this, two genetic classes of foreland basin were identified, peripheral foreland

basins and retro-arc foreland basins. Special reference is made in this chapter to the definition

of a peripheral foreland basin.

Peripheral foreland basins such as the Indo-Gangetic basin (Burbank, Raynolds & Johnson,

1986) and North Alpine molasse basin (Pfiffner, 1986) are situated against the outer arc of an

orogen during continent-continent collision brought about by A-type subduction of Bally &

Snelson (1980) (Fig. 2.2).

CHAPTER TWO


FORELAND BASINS

Page 2-2

In this context foreland basins were summarised by Allen, Homewood & Williams (1986) as

a class of structural basins which are positioned on continental lithosphere and are associated

with major compressional zones of deformation.

The main aspect of the foreland basin model is that lithospheric downflexure is caused by

thrust stacking and loading as a consequence of orogenic compression (Fig. 2.3). Different

models have been proposed for the precise mechanism of downflexure (Watts, Karner &

Steckler, 1982; Walcott, 1970 & Beaumont, 1981). The relationship between thrusting and

basin subsidence has also been investigated (Karner & Watts, 1983).

Peripheral foreland basins deepen towards the orogenic load and shallow towards the foreland

as a result of the inverse proportionality between the amount of downflexure and distance

from the load (Kominz & Bond, 1986).

CHAPTER TWO


FORELAND BASINS

-3

The close link between subsidence in a peripheral foreland basin and tectonic compression

stated above was also investigated in South Wales by Kelling (1988). This chapter

summarises these stratigraphic interpretations. Other analogous comparisons made between

foreland basin models and the Variscides of NW Europe (eg Franke & Engel, 1982) are

brought to attention as a line of regional evidence for peripheral foreland basin evolution.

This chapter will show that such comparisons depend on the identification of the main

elements of the foreland basin model in a region: the thrust load, peripheral foreland basin

and peripheral upwarp (peripheral bulge) (defined below). Complications in the Bristol

Channel area such as the occurrence of another Silesian basin in the hinterland, the Culm

Basin of North Devon (Chapter 3), instead of a thrust load need further explanation than that

offered by these previous basic models. This chapter follows the new line of reconstructions

proposed by Gayer and Jones (1989).

2.2 DEFINITION OF THE ELEMENTS OF THE FORELAND BASIN MODEL

THRUST LOAD

The thrust or nappe load (Fig. 2.4) is a structurally thickened unit of continental lithosphere

which is composed of the internal crystalline zones of an orogen and the external sedimentary

fold-thrust-belts. It is argued by Giese (1983) that the load forms by crustal stacking within

the orogen due to continent-continent collision and is situated above the underthrust

continental plate on the hinterland margin of the peripheral foreland basin. The thrust load

depresses the underthrust plate by visco-elastic or elastic mechanisms (discussed below) to

form the peripheral foreland basin. It supplies immature coarse clastic sediment to the basin

as proposed by Kelling (1988) for the South Wales Coalfield. The effective thrust load (Fig.

2.4) may be local in geographic extent as envisaged for the Bristol Channel Landmass (Gayer

& Jones, 1989) or composite, that is, spanning orogenic zones as suggested here. The effect

of sediment loading also cannot be ignored.

PERIPHERAL UPWARP (PERIPHERAL BULGE)

The peripheral upwarp (Fig. 2.4) is a positive topographic feature which migrates towards the

foreland craton. It forms as an integral response to lithospheric downflexure and acts as a

mature sediment source for the foreland margin of the basin. nb the peripheral bulge can be

geophysically defined as the first and the only significant elevated zone in a series of damped

oscillations which travel outwards from the point of loading on an elastic plate.

CHAPTER TWO


FORELAND BASINS

Page 2-4

CRATONIC LANDMASS

The cratonic landmass (Fig. 2.4) borders the foreland margin of the peripheral foreland basin.

It suffers uplift and downflexure as the thrust load, foreland basin and peripheral upwarp

migrate (discussed below). In most cases, the heterogeneities in the craton control the location

and subsidence of the basin. This is emphasised with examples from the tectonic evolution of

the South Wales Coalfield.

2.3 MECHANISM OF BASIN FORMATION AND SUSBSIDENCE

2.3.1 GENERATION OF PERIPHERAL FORELAND BASIN STRATIGRAPHY

The large scale geometry of foreland basin stratigraphy is of wedge shaped units. The wedges

thicken towards the orogenic load and thin onto the foreland to form a feather edge (Fig. 2.5).

This reflects the lateral gradient in subsidence rate from the centre of the load to the

peripheral bulge (Allen & Allen, 1990 after Kominz & Bond, 1982, 1986).

CHAPTER TWO


FORELAND BASINS

Page 2-5

Superimposed on the feather edge geometry is the movement of stratigraphic formations in

front of the advancing thrust load (Fig. 2.6). Mobility is due to the continued plate

convergence being accommodated within the orogenic belt. The result is a regional onlap of

successively younger formations on to the foreland (Wiltscko & Dorr, 1983; Allen & Allen,

1990).

Subsidence rates accommodating the sediment pile correspond with predicted values deduced

from thrust and sediment loading (Homewood, Allen & Williams, 1986) which supports the

foreland basin model (Fig. 2.7). Royden & Karner (1984) however have found discrepancies

which open a detailed debate on the correlation between thrust propagation and subsidence

curves for peripheral foreland basins (Fig. 2.7).

CHAPTER TWO


FORELAND BASINS

Page 2-6

Further complications in the stratigraphy occur due to the lateral migration of the peripheral

upwarp (Price & Hatcher, 1983). Uplift in the upwarp is succeeded by subsidence as the

upwarp progresses into the craton. Consequent erosional unconformities form which may also

migrate with time onto the craton (Fig. 2.8).

2.3.2 FACTORS CONTROLLING THE DOWNFLEXURE OF THE FORELAND PLATE

Allen & Allen (1990) stated that the deflection of the foreland plate is dependent upon the

occurrence of pre-existing heterogeneities, the flexural rigidity of the flexed lithosphere and

the nature and distribution of the thrust loads.

Pre-existing heterogeneities are especially significant in the Bristol Channel Borderlands.

They are thought to underlie the whole area as a series of basement lineaments (defined in

section 2.7.2.) of various trend (Fig. 2.9):

Lineament Trend

• Malvernoid N/S

• Caledonoid NE/SW

• Devonoid-Variscoid E/W

• Charnoid NW/SE

CHAPTER TWO


FORELAND BASINS

Page 2-7

Faults formed during previous episodes in the Wilson cycle of a plate are stated to control the

plate's behaviour during downflexure (Stockmal, Beaumont & Boutilier, 1986).

The mechanism of downflexure and the behaviour of continental lithosphere have been

investigated by Watts, Karner & Steckler (1982), Walcott (1970) and Beaumont (1981).

Watts, Karner & Steckler (1982) proposed an elastic model (Fig. 2.10) and Beaumont (1981)

suggested a visco-elastic model (Fig. 2.10). Application of the models worldwide has resulted

in, eg, the evolution of the Cretaceous foreland basin, W USA being explained in terms of

flexural response of an elastic lithosphere (Jordan, 1981) and others such as the South Wales

coal basin being explained by visco-elastic behaviour (Kelling, 1988).

Examining the detailed stratigraphy of a basin, Allen & Allen (1990) postulated that

offlapping relationships in a narrowing foreland basin may correspond to stress relaxation in a

visco-elastic lithosphere (after Tankard, 1986) (Fig. 2.10). However they extended the

argument against visco-elastic behaviour (after Sinclair et al, 1990; Flemings & Jordan, 1989)

by suggesting that similar stratigraphic geometries may be produced by a combination of

thrust load thickening above an elastic plate (Fig. 2.10). Phases of marginal uplift

immediately following loading documented by Quinlan & Beaumont (1984) however is

evidence for viscous relaxation of weakened lithosphere and have been documented from the

east crop of the South Wales Coalfield.

CHAPTER TWO


FORELAND BASINS

Page 2-8

Allen & Allen (1990) concluded that sequence boundaries may not directly reflect the exact

rheology of the flexed plate but may immediately represent episodes of load thickening or,

furthermore, quiescence in load propagation, suggesting that the tectonostratigraphy of a

peripheral foreland basin is too complex to define the lithospheric rheology.

2.4 OROGENIC ZONATION AND MODEL

2.4.1 ZONATION OF THE VARISCIDES

The Variscan orogenic belt (Fig. 2.11) (Read & Watson, 1975; Anderton et al 1979; Rast,

1983) has been subdivided into a number of zones initially based on lithological and

structural criteria (Kosmat, 1927). The zones have since been classified in relation to their

regional orogenic setting (Franke & Engel, 1982) (Fig. 2.12). This allows an unparalleled

opportunity to describe the zones in relation to their tectonic evolution (Franke, 1987). In

particular, the zones can be correlated with the tectonic elements which bound a peripheral

foreland basin. On passing from the axis of the Variscan orogen towards its northern foreland,

the following zones are traversed:

CHAPTER TWO


FORELAND BASINS

Page 2-9

• Moldanubian zone

• Saxothuringian zone, Mid German Crystalline Rise

• Rhenohercynian zone

• Sub-Variscan foredeep.

The Moldanubian zone (Suk & Weiss, 1981; Giese et al, 1983; Behr et al, 1984) (Appendix

2.1) comprises the internal metamorphic and igneous crystalline basement which in

Czechoslovakia has been affected by regional strike-slip deformation (Kachlik, Kribek, Pesek

& Rajlich, pers. com., 1990). Parts of this zone may represent fragments of an (African)

southern continental craton which docked against a (Eurasian) northern craton.

The Saxothuringian zone (Schwab & Mathé, 1981; Behr et al, 1984; Franke, 1984; Franke &

Engel, 1986; Appendix 2.2) consists of igneous intrusives and deformed Upper Palaeozoic

strata which in Germany form part of the internal Variscan thrust-fold-belt. This zone may

represent the tectonised peripheral internides which contain deformed, allochthonous

sedimentary basins and remnants of the northern craton.

CHAPTER TWO


FORELAND BASINS

Page 2-10

The Mid German Crystalline Rise (Giese et al, 1983; Behr et al, 1984; Holder & Leveridge,

1986; Fig. 2.13; Appendix 2.2) consists of Lower Palaeozoic basement and igneous intrusives

which extend between Germany and Bohemia and separate the internal zones from the

Variscan externides. The rise may represent an uplifted landmass of the northern craton or

Moldanubian Zone and may be a direct analogue for the Bristol Channel Landmass of

Tunbridge (1986) and Gayer & Jones (1989).

The Rhenohercynian zone (Engel & Franke, 1983; Franke & Engel 1986, 1988; Holder &

Leveridge, 1986; Appendix 2.3) represents the external zone of the Variscides and consists of

chevron folded and thrusted Upper Devonian-Silesian flysch basins which may represent

displaced foreland basins (thrust sheet top basins) which have been described by Gayer &

Jones (1989) and Warr (in press) in SW England and by Besly (1988) as internal and

peripheral basins (see Fig. 2.4).

CHAPTER TWO


FORELAND BASINS

Page 2-11

Towards the craton, previous peripheral foreland basins which have been deformed and

incorporated into the foreland-directed thrusting are here expected to mark the broad

boundary between the front of Variscan deformation and the foredeep constituted by

remnants of deformed peripheral foreland basins set on the uplifted northern cratonic massif

(Fig. 2.14).

The Sub-Variscan foredeep (Leeder & McMahon, 1988; Besly, 1988) (Appendix 2.4) consists

of basins set along the northern periphery of the Variscan orogen. These may represent the

youngest peripheral foreland basins generated by Variscan thrusting which mark the limits of

non-inversion controlled deformation. To the north of the Silesian basins is the northern

continental landmass consisting of Precambrian-Caledonian basement and Lower Palaeozoic

strata which form the Wales-Brabant massif. Further to the north, inversion-controlled basins

constitute the remaining foredeep.

2.4.2 VARISCAN ZONATION AND THE FORELAND BASIN MODEL

Further internal and coal-bearing Variscan zones south of the Moldanubian zone traverse

Iberia (Julivert, 1981; Savage, 1981; Ribeiro, 1981; Andrews, 1982) and the Mediterranean

(Carmignani et al, 1981; Atzori et al, 1984) and suggest that the Variscan orogen had an

element of bilateral symmetry.

The tectonic elements forming the northern segment of the orogen are of particular interest to

the evolution of the Bristol Channel Borderlands and no attempt is made to synthesise a

model for the whole European Variscides. The distribution and nature of the northern

orogenic zones represent a framework containing a variety of basins characteristically formed

during continent-continent collision (Besly, 1988).

CHAPTER TWO


FORELAND BASINS

Page 2-12

The tectonised internal zones of the orogen may represent the composite thrust load which

had significantly downflexed the northern craton to form the outwardly younging series of

Upper Devonian-Silesian foreland basins, (Fig. 2.15).

Pre-orogenic lineaments could have been active throughout the collision to form uplifted

blocks such as the Mid German Crystalline Rise and the Bristol Channel Landmass. It is

possible that the composite load consisting of crystalline basement and allochthonous basins

during the latter stage of orogenesis loaded the northern craton in conjunction with local

landmasses to form the peripheral foreland basins situated between the Rhenohercynian zone

and the northern craton. In this hypothetical case, the local landmasses and pre-existing

heterogeneities would have controlled the precise location of the peripheral foreland basins.

CHAPTER TWO


FORELAND BASINS

Page 2-13

Though simplistic, this summary of the zonation of the Variscan orogen shows that the belts

can be described in terms of the formation and movement of the elements of a peripheral

foreland basin model. It is now intended that the evolution of the tectonic elements be

described further to explain the control they exerted on the local stratigraphy of the peripheral

foreland basin South Wales Coalfield.

2.5 THE FORELAND BASIN MODEL IN SOUTH WALES

The South Wales Coalfield is situated between the thrust-belt of the Cornubian

Rhenohercynian zone and the cratonic peripheral upwarp of the Wales-Brabant massif, St.

George's Land (Kelling, 1988).

As stated previously, a basin located between a thrust load and craton is now known as a

foreland basin (Allen, Homewood & Williams, 1986). Kelling (1988) subdivided the Upper

Carboniferous stratigraphy of South Wales Coalfield into two major tectonostratigraphic

cycles (Fig. 2.16):

• Namurian - Westphalian B

• Westphalian C - Stephanian.

The cycles were related to peripheral foreland basin evolution prior to the final Variscan

deformation phase, which is open to reconsideration in view of the observation by Allen &

Allen (1990) that sequence boundaries may not directly reflect lithospheric rheological

responses. Kelling (1988) reiterated that the South Wales basin was influenced by pre-

existing structures such as the Usk-Malvern axis, Caledonide elements (Owen & Weaver,

1983) and southerly lineaments (Kelling, 1974). These complicated the sequence stratigraphy

further.

However palaeogeographic analysis indicates that marine influence persisted longer to the

south and south-west in the South Wales basin (Kelling, 1974; Thomas, 1974). Basin

constriction also began in the south based on sedimentology by George & Kelling (1982).

These observations broadly compare with a foreland basin setting.

CHAPTER TWO


FORELAND BASINS

Page 2-14

The initial Namurian-Westphalian B phase of basin history can be summarised as an episode

of basement fault reactivation controlling the thickness and type of syn-orogenic

sedimentation.

Kelling (1988) emphasised the major marine incursion represented by the G. Cambriense,

Upper Cwmgorse marine band at the boundary between the Middle and Upper Coal

Measures. This boundary represents the change from an early reactivation-dominated history

to a late thrust-dominated history represented sedimentologically by the Late Westphalian-

Stephanian coarse lithic detritus derived from the encroaching southern tectonic landmass.

The sedimentological consequence of the change in structural environment was a replacement

of high sinuosity fluvial, deltaic and marginal marine facies by a regional low sinuosity

alluvial complex.

The two-fold subdivision of the South Wales Silesian was interpreted by Kelling (1988) in

the light of the tectono-sedimentary evolution of a peripheral foreland basin postulated by

CHAPTER TWO


FORELAND BASINS

Page 2-15

Beaumont (1981) and Hayward (1984) which involves basin growth and filling associated

with thrust loading and responsive lithospheric down and up-warping. The model includes the

lateral migration of the foreland basin following the later phase of thrust emplacement (Fig.

2.17).

The Namurian-Early Westphalian resulted from the initial thrust loading and peripheral

upwarping which enhanced erosion of the foreland margin and eastern marginal areas.

Kelling (1988) suggested that the initial response to loading was visco-elastic upwarping of

the cratonic edge of an underdepressed basin followed by visco-elastic relaxation as a result

of the initial emplacement of the thrust load (Quinlan & Beaumont, 1984).

CHAPTER TWO


FORELAND BASINS

Page 2-16

The Late Westphalian-Stephanian resulted from enhanced loading and by northward advance

of nappes shifting the peripheral foreland basin and peripheral bulge further onto the craton

(Fig. 2.18).

Hence in terms of its present zonal and orogenic setting the South Wales basin is a classic

foreland basin in a peripheral position to major thrusts of SW England. However the precise

relationship between SW England and South Wales during the Silesian has not been

constrained to allow a quantitative assessment of lithospheric behaviour. Therefore the exact

mechanism of basin formation is still theoretically open to debate. Quantitative stratigraphic

analysis would assist in comparing the basin history and the geophysical models of basin

subsidence, (eg Kelling, 1988; Jones, 1989; Fig. 2.19). However greater biostratigraphic

resolution and better correlations with SW England are essential requirements (but are

beyond the scope of this thesis).

2.6 FORELAND BASIN MODEL AND THE RHENOHERCYNIAN ZONE

The regional approach in describing the Variscan orogenic zones as a series of belts formed

by foreland basin processes has been attempted cursorily in sections 2.4.1 & 2.4.2. The

significant discovery in the regional aspect of the Variscides is the similarity of sections

through the zones in Germany and SW England (Fig. 2.20).

CHAPTER TWO


FORELAND BASINS

Page 2-17

The German Variscides have been described in relation to peripheral foreland basin tectonism

and provide a good analogue for the structural zonation of the Variscides of SW Britain.

Although the zonation and correlation of the Variscan zones may be inconsistent across

CHAPTER TWO


FORELAND BASINS

Page 2-18

Europe, the subdivision of continental basins into internal and external peripheral foreland

basins is a revealing classification which is here applied to the North Devon Culm Basin and

the South Wales Coalfield.

Following this line of argument, the elements of peripheral foreland basin tectonics, the thrust

load and its internal basins, the foreland basin and the cratonic peripheral upwarp are all

located within the Bristol Channel Borderlands (see Figs. 2.4 & 2.21).

Franke & Engel (1988) described a number of zones which included an internal thrust sheet

with internal allochthonous basins separated from the autochthonous foreland basin by a

landmass or uplifted massif. This is directly analogous to the situation in SW England where

the allochthonous thrust sheet and internal or peripheral basins (Besly, 1988) are found in SW

England, such as the Culm thrust sheet top basin of Gayer & Jones (1989), to be separated

from the autochthonous South Wales coal basin (Kelling 1988) (section 2.5) by a postulated

Bristol Channel Landmass (Fig. 2.21; see Chapter 3).

Though there are problems in making direct correlations of the zones across Europe due to

postulated strike-slip faulting (Holder & Leveridge, 1986) the similarity in structure remains

striking.

However, as an example to illustrate the problem of correlating the zones across the

Variscides the location of the orogenic load is questioned. In the study area it is expected to

lie to the south of the peripheral foreland basin. The apparent anomalous occurrence of

another Upper Palaeozoic basin, the Culm basin, instead of a load stands as good evidence

against the peripheral foreland basin model. This argument stands until the possibility of a

composite load is raised or the possibility of a hidden load is investigated (Chapter 6).

2.7 SEQUENCE STRATIGRAPHY AND BASEMENT LINEAMENTS

2.7.1 SEQUENCE STRATIGRAPHIC GEOMETRIES IN THE BRISTOL CHANNEL

BORDERLANDS

Major faults beneath the Bristol Channel are suggested here to have played an integral part in

the structural development of the orogenic load (Chapter 6). Together with pre-existing

lineaments beneath South Wales (section 2.7.2) the final stratigraphic template of the

peripheral foreland basin of South Wales became dramatically complicated by End

Carboniferous times. This is emphasised in the present study of unconformities in the Bristol

Channel Borderlands.

CHAPTER TWO


FORELAND BASINS

Page 2-19

The sequence stratigraphic geometry expected in a foreland basin has been discussed in

section 2.3.1. The following maps (Figs. 2.22 - 2.25) illustrate the main elements of the

stratigraphy of a peripheral foreland basin: the feather edge; onlap; foreland-migrating

unconformities. Examples of local unconformities formed in the Bristol Channel Borderlands

during the evolution of the South Wales foreland basin illustrate the regional history of the

South Wales basin discussed in section 2.5 and possibly the tectonic processes summarised in

section 2.3.2.

CHAPTER TWO


FORELAND BASINS

Page 2-20

2.7.2 BASEMENT LINEAMENTS

All the basement lineaments listed in section 2.3.2 are here thought to have affected

peripheral foreland basin stratigraphy in the Bristol Channel Borderlands. The oldest

movement is unknown for all the lineaments. However they have been named after a region,

or the oldest age or orogen in which they are known to have moved or developed

significantly, based on structural and stratigraphic evidence from the survey area.

CHAPTER TWO


FORELAND BASINS

Page 2-21

MALVERNOID LINEAMENTS (N/S - NNE/SSW)

It is here suggested that the Malvernoid lineaments may have bounded a basement high

during early peripheral foreland basin development. This may have represented a peripheral

bulge oblique to the direction of thrust advance. As the thrust sheet approached it is possible

that more lineaments were reactivated to produce a composite peripheral upwarp. The

Malvernoid lineaments may have been active during a marginal phase of uplift (based on

Kelling, 1988).

CALEDONOID LINEAMENTS (NE/SW)

These are Caledonian faults which may also have a Precambrian history. In SW Dyfed the

trend swings clockwise into an ENE/WSW orientation, however it is the occurrence of

NE/SW disturbances within the South Wales basin that indicates syn-orogenic sedimentation

was controlled in part by Caledonoid basement fault reactivation (Owen & Weaver, 1983).

DEVONOID-VARISCOID LINEAMENTS (E/W - ESE/WNW)

Devonian extensional movement histories have been proved by Powell (1987) on E/W

trending Variscan thrusts in SW Dyfed. This is the oldest, direct evidence of movement on

the lineaments, however seismic evidence from the Bristol Channel suggests that an ESE

trending Devonoid fault extends to depths at which crystalline basement is anticipated from

seismic refraction surveys. Such faults may have become active during Variscan thrust sheet

advance to produce uplifted blocks such as the Bristol Channel Landmass which deepened the

peripheral foreland basin to the north and locally supplied sediment to the basin. nb Many

east-west trending faults are pristine Variscan structures.

CHARNOID LINEAMENTS (NW/SE - NNW/SSE)

Charnoid lineaments, similarly to the Malvernoid lineaments, may have bounded syn-

orogenic basement highs which controlled the thickness of foreland sediments by extension

(Hancock & Bevan, 1987). Some also show a Late Variscan movement which

compartmentalises the structure of the Bristol Channel Borderlands but may have a

Precambrian origin.

2.8 CONCLUSIONS

It is questionable whether the Upper Palaeozoic stratigraphy and structure of the Bristol

Channel Borderlands were controlled solely by peripheral foreland basin tectonics. Further

research is required to quantify the effects of reactivation on synorogenic sedimentation and

late structural style.

CHAPTER TWO


FORELAND BASINS

Page 2-22

Regional examination of the Variscan zones has shown along-strike similarities that are

broadly consistent with the distribution of terranes related to a foreland basin.

Problems encountered in defining the tectonic elements in the Bristol Channel Borderlands

can be explained on considering a composite load. The composite load contains

allochthonous thrust sheet top basins and internal zones of the orogen.

The final tectonic location and late history of the South Wales Coalfield was set in a complex

intracontinental foreland setting.

CHAPTER TWO


FORELAND BASINS

Page 2-23

Appendix 2.1 Moldanubian Zone

The Moldanubian Zone is the internal zone of the

Variscan orogen and consists of high grade metamorphic

crystalline basement which is partly covered by

Precambrian-Upper Palaeozoic Barrandian sediments.

Rocks of the Moldanubian Zone were examined in the

Bohemian Massif, Czechoslovakia during September

1990 from which the following details were obtained

(after Kachlik, Kribek, Pesek and Rajlich pers. com.,

1990).

The Moldanubian Zone forms the southernmost part of

the northern branch of the Variscan orogenic belt. It

covers the French and Czechoslovak massifs and is

separated from the Saxothuringian Zone (Appendix 2.2)

to the north by the Ernberdorf-Litomerice Fault Line. In

the ESE the Moldanubian zone is thrust over the

Rhenohercynian Zone (section 2.2.3) (Appendix 2.3) and

in the south the zone is separated from the Alps by the

Brunovisticulum Lineament.

The Moldanubian Zone is composed of Moldanubian

(1000Ma) and Cadomian (650Ma) amphibolite facies

which display a Variscan greenschist facies

retrogression.

Crystalline complexes were developed during latest

Proterozoic; Caledonian (450-390Ma) and Early

Variscan times. An example of the latter is the Central

Bohemian granitoid pluton. Granulite and eclogite facies

metamorphism is also thought to have affected this zone.

In general during early Variscan times greenschist and

amphibolite facies metamorphism was accompanied by

syn-tectonic intrusive activity.

SUBDIVISION OF THE MOLDANUBIAN ZONE

The Moldanubian Zone contains the Bohemicum sub-

zone composed of unmetamorphosed, slightly deformed,

Cambrian-Devonian cover-rocks, known as Barrandian

(Prague Basin), which are separated from the main

Moldanubian Zone by the Central Bohemian Pluton

(south of Pribram). These rest on the Barrandian

Precambrian which consists of Upper Proterozoic

basement composed of oceanic sediments (Stechovice

Group) and also ophiolitic series. The ophiolitic series

are thought to be derived from oceanic crust which was

bound between the north European continent and

Gondwanan micro-continental blocks north of the

Brunovisticulum Lineament. Subduction occurred from

Brioverian to Cadomian times and generated the tholeiite

volcanics of the region.

The Moldanubian Zone s.s. occupies the southern part of

Bohemia and SW Moravia, N Austria and E Bavaria

from which rocks of the S Bohemian Massif have been

examined.

The Moldanubian Zone is bounded to the north-west by

the discrete contact with the Central Bohemian Pluton

and to the north and north-east by the Bohemian

Cretaceous unconformity. In the south the Moldanubian

Zone extends to the Danube Line and continues beneath

the Alpine Foredeep. The western boundary is marked by

the Domazlice Crystalline Complex and the eastern

boundary is a tectonic contact between the Moldanubian

Zone and the external orogenic zones.

The Moldanubian Zone is composed of two main

metamorphic-stratigraphic units, the lower Monotonous

Zeliv Group and the Upper Varied Cesky Krumlov

Group.

CHAPTER TWO


FORELAND BASINS

Page 2-24

The Monotonous Group consists of 3000m of gneiss

containing occasional intercalations of quartzite. These

are succeeded by pyroxene granulites and amphibolites

of higher amphibolite to hornblende-granulite facies.

In contrast, the Varied Group consists of 100-1000m of

gneiss, marble, graphite and metabasite succeeded by

about 2000m of mica schist and quartzite of lower

amphibolite, kyanite-staurolite facies such as in the

Kutna Hora Crystalline Complex.

STRUCTURE OF THE MOLDANUBIAN ZONE

Field examination of the Moldanubian Zone showed that

the tectonic history of the zone is complex. It is presently

thought that the zone was heavily affected by Variscan

strike-slip faulting and transpression. Compressional

structures are abundant in the crystalline complexes and

are accompanied by various indicators of shear on which

kinematic models of transpressional are based.

Here an outline of the structural style is given from

examinations of the following regions: Bohemia near

Prague; Cesky Krumlov in S Bohemia and Kutna Hora.

PRAGUE, BOHEMICUM BASEMENT

The Bohemicum Basement south of Prague has been

affected by two phases of strike-slip movement: sinistral

strike-slip was succeeded by NE dextral strike-slip, N/S

extension and granite intrusion. The dextral phase is

interpreted as transtensional. Evidence for dextral shear

was observed in the Stechovice agglomerates.

CESKY KRUMLOV, VARIED GROUP

N/S extensional structures were observed in granulite of

the Varied Group of Cesky Krumlov as part of three

phases of deformation. The early main phase however

involved dextral shear along ESE striking zones which

was syn-amphibolite metamorphism. The later Variscan

phase caused thrusting to the SE which was syn-

retrograde metamorphism and granitisation and the latest

phase formed boudinage indicating various directions of

extension.

In the Varied Group of Slavkov the Variscan thrusting

phase affected the rock most strongly. Isoclinal folding

trending N/S to NE/SW is associated with the thrusting

to the SE away from the Krumlov Shear Zone. Boudins

also show a related NW/SE stretching axis of 150°.

Variscan folds also affect the Metsky vrch Hill graphite

deposits. However here fold axial traces trend N/S with

synclines plunging moderately towards the south.

KUTNA HORA, CRYSTALLINE COMPLEX

Pencil gneiss with associated sheath folding from Kutna

Hora preserve a phase of E/W extension associated with

sinistral strike-slip along the E/W trending Elbe Shear

Zone. This was accompanied by E/W extension along

N/S striking faults. The E/W extension is thought to have

been caused by top to the west shear along shallow angle

shear zones. Evidence of top to the west shear occurs in

highly strained Devonian conglomerates at Branna.

SYNOPSIS OF THE MOLDANUBIAN ZONE

Evidence has been given to suggest that the Moldanubian

Zone consists of Precambrian crystalline complexes

containing Variscan intrusions. The crystalline basement

displays abundant evidence of shear associated with

regional Variscan strike-slip which contrasts with the

shallow compressional structural style in the Variscan

externides.

CHAPTER TWO


FORELAND BASINS

Page 2-25

INFERENCE

Examination of intense strike-slip deformation in the

Moldanubian Zone suggests that Late Palaeozoic

collision between Gondwana and Eurasia may have been

oblique. Further research is needed to evaluate Variscan

internide strike-slip deformation in relation to thrusting

in the externides.

Appendix 2.2 Saxothuringian Zone and the Mid

German Crystalline Rise

The Saxothuringian Zone, the outer internal zone of the

Variscides, lies between the Moldanubian Zone in the

south and the Mid German Crystalline Rise in the north.

The zone was examined in the Taunus region, Germany

during September 1988 where it consists of Variscan

granitic intrusives, lavas and Devonian greenschist facies

metasediments.

In contrast, the Mid German Crystalline Rise consists of

Upper Proterozoic-Ordovician metasediments, volcanics

and pre-Variscan granitic intrusions.

The Saxothuringian Zone extends through the Thuringer

Wald, Saxony, N Vosges, Schwarzwald into the

Bohemian Krusne hory and Orlicke hory Mountains. The

Mid German Crystalline Rise also extends eastwards

around the NE periphery of the Bohemian massif where

it is thrust eastwards on to the Rhenohercynian Zone.

As an example of the geology of the Saxothuringian

Zone the stratigraphy and structure of the Taunus Region

is given below (Greiling, pers. com, 1988).

STRATIGRAPHY OF THE TAUNUS REGION

The greenschist facies metasediments of the Taunus

sequence consist of lowermost Devonian phyllites

succeeded by interbedded slaty pelites and psammites

and eventually by the planar and cross-bedded Taunus

Quartzite of Early Devonian, Siegenian age. The

quartzite is succeeded to the north by further

metasediment such as the Hartzenheim Phyllite.

STRUCTURE OF THE TAUNUS REGION

There is good structural evidence to suggest that the

Saxothuringian Zone was affected by Variscan thrusting.

The Devonian rocks of the Taunus region have

undergone two major phases of deformation (1&2)

followed by a third minor phase (3). These rocks of the

Saxothuringian Zone have developed a pervasive

phyllitic and slaty cleavage found to be bed parallel in

the less metamorphosed rocks. The cleavage dips

moderately towards the SE and is folded by late minor

folds, some of which are parasitic to decametre folds

plunging gently towards the WSW.

The late folding (2) commonly produces axial planar

cleavage which dips gently towards the NW though some

axial planes dip moderately towards the SE. Cleavage

fanning is also locally observed in upright folds with

axial planes striking NE/SW. The early cleavage (1) is at

a moderate angle to bedding. Kink bands deform late

fold axial surfaces (2) and early slaty cleavage (1) and

clearly represent the latest phase of minor deformation

(3).

In general, on passing northwards through the zone into

the Rhenohercynian Zone of the Taunus Region the

attitude of late cleavage (2) in relation to early planes (1)

is found to change in the following manner: gently south

dipping late cleavage passes N into horizontal and gently

north dipping cleavage and eventually into steeply

CHAPTER TWO


FORELAND BASINS

Page 2-26

dipping late cleavage where early cleavage and bedding

have gentle dips.

SYNOPSIS OF THE SAXOTHURINGIAN ZONE

These sediments may represent pre-orogenic

supracontinental deposits north of the main orogenic

suture which have been preserved in an internal basin

setting as part of a foreland thrust sheet after Variscan

deformation (based on Besly 1988). The decrease in

regional metamorphism suggests a peripheral internide

location for the Saxothuringian Zone in relation to the

Moldanubian Zone. The Mid German Crystalline Rise

however may represent an uplifted foreland basement

block with greater affinity to the internal Moldanubian

Zone or the craton to the north. A substantial fault

displacement is inferred to have caused the Variscan

uplift of the rise along the periphery of the

Saxothuringian Zone.

Thrusting in the Saxothuringian Zone contrasts with the

shear-dominated deformation of the Bohemian Massif.

The Saxothuringian Zone may have had characteristic

effects on the stratigraphy, sediment type and supply to

the Rhenohercynian Zone due to its proximity and style

of thrust deformation. However the setting is

complicated by the Mid German Crystalline Rise which

may have acted as a barrier to sediment supply from the

Saxothuringian Zone into the Rhenohercynian Zone and

may also have complicated the thrust loading of the crust

(sections 2.3.2 & 2.4.2).

Appendix 2.3 Rhenohercynian Zone

The Rhenohercynian Zone forms the northern, external,

marginal zone of the Variscides and consists of

tectonised Upper Palaeozoic strata which flank the

hinterland margins of the peripheral foreland basin

coalfields (defined in sections 2.1 & 2.5).

The Rhenohercynian Zone extends from N Devon,

Ardennes, Rheinische Schiefergebirge and N Harz,

Germany into the Moravo-Silesian region of the

Bohemian Massif. The zone has been examined in N

Devon, (Taunus, Lahn Valley, Sauerland, Eifel)

Germany and the Hruby Jesenik Mountains in

Czechoslovakia.

Here a summary of the stratigraphic age, style of

sequence and structural character is given as a regional

comparison with the Rhenohercynian Zone of N Devon

(after Holder & Leveridge, 1986).

SYNOPSIS OF THE RHEINISCHE

SCHIEFERGEBIRGE

The main features of the stratigraphy of the Rheinische

Schiefergebirge are that the Devonian consists of a

mixed siliciclastic/carbonate sequence which has greatest

affinity to the marine Devonian sequences of South

Devon. These are succeeded by Lower Carboniferous

black chert similar to that found along the northern

margin of the Culm Basin (Chapter 3). There is a noted

absence of a Lower Carboniferous carbonate platform

such as that found in South Wales.

The chevron style folding is very similar to regional

Culmian structure of the Rhenohercynian Zone. The

structural trend in the Rheinische Schiefergebirge is NE-

SW in contrast to the E-W trend in the Bristol Channel

Borderlands. Thrusting is directed towards the NW and it

is here thought that the change in structural trend is

probably a primary feature of the zone which has been

complicated in parts of Germany by the modifying effect

of the Wales-Brabant massif, Mid German Crystalline

CHAPTER TWO


FORELAND BASINS

Page 2-27

Rise and associated basement fault reactivation.

Movement within such basement blocks may have been a

major modifying factor in the propagation of the

Variscan structural wave front.

SYNOPSIS OF THE JESENIK MOUNTAINS

Culmian flysch deposits comparable to those of N Devon

(Chapter 3) have been folded into chevrons which are

directly comparable to the folds in the Culm Basin. The

anomalous N-S trend and vergence to the east is a

dramatic contrast to the E-W trend in the Bristol Channel

Borderlands and NE-SW trend in Germany. This is also

thought to be a primary feature of the orogen and is here

thought to be due to regional strike-slip re-orientation of

the foreland due to oblique orogenic collision. The onset

of flysch sedimentation began in Early Carboniferous

times and continued until the Late Carboniferous when it

was succeeded by paralic sedimentation. This contrasts

with the mainly Namurian and Lower Westphalian age of

the formations in the Culm Basin. The onset of the

orogeny in the east is here interpreted to have been

earlier than in the Bristol Channel Borderlands though

the structural regime was practically identical causing a

regional shortening of about 45% comparable to the

shortening measured in the Culm Basin of 50%

(discussed in Chapter 3).

APPENDIX 2.4 SUB-VARISCAN FOREDEEP

Three coalfields have been examined which lie along the

southern margin of the Sub-Variscan Foredeep: the Ruhr

and Ostrava Coalfields under reconnaissance and the

South Wales Coalfield (Chapter 4).

To the north of the Rhenohercynian Zone generally lies

the Sub-Variscan Foredeep which consists of less

tectonised Upper Palaeozoic coal-bearing strata

deposited in extensional basins (Leeder and McMahon,

1988) which partly experienced inversion due to the

distant effects of Variscan orogenic collision.

However on examining the coalfields lying adjacent to

the Rhenohercynian Zone Variscan structural complexity

is found to continue into them.

The coalfields show differing trends (E/W, South Wales;

NE/SW, Germany; N/S, Czechoslovakia). However

thrust and fold development is generally towards the

foreland. Examples of thrusts from the Ruhr Coalfield

indicate a NW direction of transport whereas those of the

Ostrava Coalfield show an E direction of transport with

deformation decreasing towards the east. This contrasts

strongly with the N-directed thrusting in the South Wales

Coalfield in which deformation along the north crop is

still intense (discussed in Chapter 4).

CHAPTER TWO


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Page 2-28

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FIGURE CAPTIONS

Fig. 2.1 Inset showing the distribution of foreland basins, a. the location of the study area and b. a type area, illustrated in a

simplified geological map: the European Alpine System showing the main tectonic units. 1. Crystalline pre-alpine plutonic

and metamorphic basement rocks and ophiolites, ultramafics, blueschists and eclogites. 2. Crystalline pre-alpine rocks of the

external massifs. 3. Continental shelf type sedimentary rocks. 4. Sediments of the Molasse basin and post-orogenic

sediments.

Fig. 2.2 Formation of a peripheral foreland basin by the flexural bending of cratonic lithosphere as a result of continental

collision brought about by A-type subduction.

Fig. 2.3 Illustrations of models of crustal thickening, a. thickening by thrust stacking within the crust and b. thickening by

block faulting and associated vertical accretion at the base of the crust.

Fig. 2.4 Schematic section through an intracontinental mountain belt showing the elements of the peripheral foreland basin

model: the cratonic peripheral upwarp; peripheral foreland basin; local thrust load and regional composite load; thrust sheet

top basin (satellite basin). The vertical scale is greatly exaggerated (about x10).

Fig. 2.5 Illustration of the stratigraphic pattern of onlap in a foreland basin, onto the foreland plate. T1-T4 represent

successive chronostratigraphic lines emphasising the feather edge geometry of the foreland basin sequence.

Fig. 2.6 Quantitative stratigraphic graphs, in support of the foreland basin model, representing subsidence rates that

correspond with thrust and sediment loading rates. Plot a. shows the successive pinch out of stratigraphic units and plot b.

shows the migration of postulated depocentres on restored sections by Homewood, Allen & Williams (1986). Plots a. & b.

indicate rates of convergence during the Oligo-Miocene. For comparison, a distance-time graph for neo-alpine deformation

in Western Switzerland showing: c. the tip propagation rate and d. the shortening rate assuming a 50% shortening across the

belt.

Fig. 2.7 Graphs showing discrepancies between correlations of thrust loading and subsidence associated with peripheral

foreland basins: transects across the Apennine thrust belt and Adriatic foreland showing topography (in black) and observed

thickness of Pliocene to Recent sedimentary rocks within the foredeep trough (heavy line). a. shows the contrast between an

approximate curve representing the calculated deflection of the lower Adriatic plate for various elastic plate thicknesses

(thin line) and the curve representing change in actual sediment thickness. b. shows an apparently good correlation between

subsidence and loading assuming the presence of a significant, additional, vertical, line force.

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Page 2-33

Fig. 2.8 Truncation of unconformities due to the cratonward migration of the peripheral upwarp causing episodes of uplift

being followed by subsidence. The arrow shows the direction of movement of the peripheral upwarp. uc1, uc2:

unconformities. (Based on Allen & Allen, 1990 after Tankard, 1986).

Fig. 2.9 Sketch map of the eastern part of the Bristol Channel Borderlands showing selected major faults which have

undergone reactivation. m. Malvernoid, c. Caledonoid, d. Variscoid-Devonoid, v. Charnoid. These faults may be basement

related.

Fig. 2.10 Illustrations of the flexural behaviour of the lithosphere in the case of a. the Elastic Model and b. the Visco-elastic

Model. 2.10b. shows a cross sectional view of the surface deformation of a continuous visco-elastic lithosphere under a

surface load. The initial response, stage 1, is the same as that shown in 2.10a. As time progresses relaxation of stress makes

the profile evolve through stages 2 & 3 as the response progresses toward local isostatic equilibrium (Quinlan & Beaumont,

1984).

Fig. 2.11 Sketch map of the Variscan belt and associated belts. a. Appalachian belt; lc Laurasian continent; sc Siberian

continent.

Fig. 2.12 Sketch map showing the distribution of the Variscan orogenic zones. RZ Rhenohercynian Zone; SZ

Saxothuringian Zone; MZ Moldanubian Zone.

Fig. 2.13 Inset showing a location map of the Mid German Crystalline Rise (mGCR) in relation to the restored Variscan

orogenic zones: A Armorican; CI Central Iberian; m Mediterranean; RZ Rhenohercynian; SZ Saxothuringian; MZ

Moldanubian. (Sub-zone: PZ Phyllite Zone; localities: t Taunus, h Harz, v Vosges, sw Schwarzwald). The Mid German

Crystalline Rise is composed of Variscan plutonic rocks and metamorphic rocks bounded to the south by pre-Devonian

partly metamorphosed rocks.

Fig. 2.14 Schematic cross section through a thrust deformed hinterland h., foreland and remnant basin pfb., set on an

uplifted craton u cr. The intracontinental basin is separated from the foreland by an uplifted block.

Fig. 2.15 Inset map 2.15a represents the distribution of Variscan massifs in Europe (in black) which have been subdivided

to produce the tectonic map 2.15b representing the major tectonic elements in the northern section of the Variscides.

Localities: bcb Bristol Channel Borderlands; s Stockholm; p Paris; pr Prague; b Budapest; k Kisinov. KEY: v - shaped

ornament Northern Craton; stippled ornament Variscan Foredeep; pebble ornament Peripheral Foreland Basins; small

circular ornament Crystalline Rise; wavy ornament Internal Basins; inverted v - shaped ornament Internal Crystalline

Basement.

Fig. 2.16 Synoptic tectonostratigraphy of the South Wales Coalfield. Stages: Dinant. Dinantian; Namur. Namurian;

Stephn. Stephanian. Lithostratigraphy: L&M CM Lower and Middle Coal Measures; UCM Upper Coal Measures. The

section shows the correlation between the stratigraphic succession and Variscan structural events.

CHAPTER TWO


FORELAND BASINS

Page 2-34

Fig. 2.17 Illustration of the foreland basin model applied to the South Wales Coalfield illustrating foreland basin migration

due to the advance of the thrust load.

Fig. 2.18 Model representing the growth and infill of a peripheral foreland basin. A. the load depresses a baseline (faulted)

to form the basin and a peripheral upwarp, B. The basin is filled by sediment eroded from the thrust load and the peripheral

upwarp C.

Fig. 2.19 Geohistory curves for the South Wales Coalfield (after Jones, 1989). Curve 1 (after Kelling, 1988). Stages: D

Dinantian; N Namurian; W Westphalian; S Stephanian. The curves show a dramatic increase in subsidence rate during

Westphalian times.

Fig. 2.20 Diagrammatic illustration of a section through the German Variscides, during Lower Carboniferous times, shows

the distribution of the elements of the peripheral foreland basin model, analogous to the structure of SW England.

Lithologies: ch. Chert; sh. Shale; gt. Greywacke Turbidite; lmst. Limestone.

Fig. 2.21 Location of the elements of the peripheral foreland basin model in the Bristol Channel Borderlands. Vertical

exaggeration (x10).

Fig. 2.22 Location map of the areas of stratigraphic interest in the Bristol Channel Borderlands.

Fig. 2.23 Sketch map of the Haverfordwest area showing the onlap of Upper Palaeozoic strata onto the Lower Palaeozoic of

St. George's Land. The map shows the distinct pinch out of the Carboniferous Limestone on passing westwards along strike.

This may represent an example of onlap against a topographic high predating the formation of a peripheral upwarp and

formed during upwarping. Localities: Hw Haverfordwest; Pp Picton Park; Js Johnston.

Fig. 2.24 Sketch map of the Portishead area showing the unconformable relationship between Pennant Measures and

Carboniferous Limestone. This may represent evidence for pre-Pennant deformation which was associated with the onset of

thrust advance into the area. Localities: Cl Clevedon; Ph Portishead; Av Avonmouth.

Fig. 2.25 Sketch map of the east crop of the South Wales coalfield evidently showing the thinning of Carboniferous

Limestone, Millstone Grit and Lower and Middle Coal Measures on passing towards the Malvernoid Usk axis. This suggests

the Usk area was a positive topographic feature which may be accentuated by a marginal phase of peripheral upwarping.

Localities: Cf Cardiff; Nw Newport; Ch Chepstow.

Marios Miliorizos

25th August 2005

File name: PhD Chapter 2 Two

tectonic evolution of the bristol channel borderlands chapter 2

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