tectonic evolution of the bristol channel borderlands chapter 3

CHAPTER THREE

Tectonic Evolution of the Bristol Channel borderlands

CORNWALL, NORTH DEVON, SOMERSET

Page 3-1

3. THE LATE PALAEOZOIC INVERSION HISTORY OF FAULTS IN NORTH

DEVON AND NORTH CORNWALL AND THE VARISCAN TECTONIC

EVOLUTION OF THE REGION.

3.1 INTRODUCTION

3.1.1 AIMS

The primary aim of this chapter is to demonstrate the Devonian and Carboniferous inversion

history of faults in North Devon and Cornwall. Evidence for fault inversion will be discussed

in terms of (1) regional studies of facies distribution and (2) local structural geological case

studies.

1. The regional studies will take account of hypothetical Devonian and Carboniferous

landmasses as well as Carboniferous basin formation and dynamics.

2. The local case studies will describe areas along the North Devon and Cornish coasts.

Detailed examination of representative faults within the North Devon and Culm Basins will

also illustrate the inversion history.

A second aim is to give a comparative account of the Devonian and Carboniferous history of

South Devon, North Devon and North Cornwall as a basis for later comparisons with the

geological history of South Wales (Chapter 4). This regional account will present evidence for

the nature of the offshore stratigraphy beneath the Bristol Channel and will include evidence

for the presence of major offshore Variscan structures, including north- westerly trending

faults which appear to have played a significant role in the evolution of the Bristol Channel

Borderlands. Evidence for offshore structures will also be given in Chapter 6 based on further

detailed onshore geological surveys and case studies.

A final aim of this chapter is to consider the Variscan tectonic load in relation to Variscan

basin dynamics. This load was postulated (see Chapter 2) to occur in North Devon and the

Bristol Channel and is thought to have produced the foreland basin of South Wales.

Furthermore in terms of the regional evolution it will be demonstrated how the NW trending

faults could have controlled the lateral, along-strike distribution of the Variscan load and

controlled movement and partitioned structures within the foreland basin of South Wales.

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Page 3-2

3.1.2 DEFINITIONS

The main result of investigations into the geology of North Devon is the recognition of two

Late Palaeozoic basins, the North Devon Basin (Exmoor Basin) described in part by, eg,

Goldring (1962), Tunbridge & Whittaker (1978), Durrance & Laming (1982), Edmonds et al

(1985), and the Culm Basin (Bude Basin) described by Thomas (1988) and in part by, eg,

Freshney & Taylor (1972), Edmonds et al (1979), Freshney et al (1979a, 1979b), Selwood &

Thomas (1986).

The North Devon Basin

The North Devon Basin (Fig. 3.1b) occupies the northern limb of the major synclinorium of

SW England (Fig. 3.1a) and extends on land from Morte Point, west of Ilfracombe, to the

Quantock Hills in the east (BGS 1:50 000 scale sheets 276-279 & 292-295 Ilfracombe,

Bideford & Lundy) and southwards, through Exmoor, to Barnstaple and North Molton where

it meets the northern crop of the Culm Basin (defined below). Similar rocks to those of Morte

Point also crop out on Lundy to the west (BGS 1:50 000 scale sheet 292 Bideford & Lundy).

(A)

The North Devon Basin consists of Lower-Middle and Upper Devonian strata which form a

thick siliciclastic sequence dominated by slate and fine-grained lithic sandstones. The North

Devon Basin contains the oldest rocks in the northern limb of the synclinorium and shows a

conformable stratigraphic relationship and faulted contact with overlying rocks of the Culm

Basin.

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Page 3-3

(B)

The Culm Basin

The Culm Basin, described in case studies during the later sections of this chapter, is folded

into a major synclinorium (Fig. 3.1b) and extends from the area around Instow southwards to

Rusey (BGS 1:50 000 scale sheets 307-309 & 322-324 Bude, Boscastle) where it displays a

definite faulted contact with the Trevone Basin to the south (Matthews, 1977). The axis of the

basin extends east-west from Bude to Crediton where it is marked geologically by the Crediton

Trough. The Culm Basin consists mainly of turbiditic siliciclastic sequences (eg, Mackintosh,

1964; Melvin, 1986; Fig. 3.1a) overlying a relatively thin sequence of cherts (eg, Prentice,

1960).

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Page 3-4

The detailed sedimentology of the Culm Basin has been studied by, for example, De Raaf et al

(1965), Walker (1970), Elliott (1976) & Xu Li (1990). The turbidites range from mudstone

dominated in lower parts through to thickly-bedded sandstone dominated in upper parts of the

sequence (eg, Higgs, 1984). The strata of the Culm Basin range in age from Dinantian to

Westphalian C. The latter age is significant in the evolution of the Bristol Channel Borderlands

(Gayer & Jones, 1989) in that it marks the end of Carboniferous sedimentation in the Culm

Basin and the onset of Pennant Measure sedimentation in South Wales.

3.2 REGIONAL EVIDENCE FOR STRUCTURAL INVERSION

3.2.1 INTRODUCTION

Devon and Cornwall contain a nearly complete Upper Palaeozoic succession ranging in age

from Lower Devonian to Westphalian C (Allen, 1974; Burne & Moore, 1971). However,

inspection of the relationship between fault structure and stratigraphy does not yield an

obvious inversion history for North Devon. The following sections attempt to illustrate the

structural history of the region (1) with reference to the influence of hypothetical Upper

Palaeozoic landmasses (section 3.2.2) and the location of sediments derived from them and

(2) from detailed case studies of Devonian sediments and their potential relationship with

local faulting (section 3.2.3). The review of the regional evidence for fault inversion is given

in preparation for the detailed surveys along the North Devon and Cornish coasts.

3.2.2 THE BRISTOL CHANNEL LANDMASS

Early Devonian

The Early Devonian tectonic setting of SW Britain is uncertain. However based on regional

considerations, such as the position of SW Britain in relation to End Caledonian tectonic

provinces and also prior to Variscan shortening, three possible hypotheses are (1) SW Britain

was affected by regional intracratonic extension following the Caledonian orogeny; (2) SW

Britain was affected by regional cratonic extension, eg near a passive margin, unrelated to

extension in South Wales (3) SW Britain was situated on the cratonic side of a tectonically

stable passive margin.

The earliest general evidence of Late Palaeozoic tectonism in Devon is the onset of fluvial

deposition of the Middle Dittonian to Breconian Dartmouth Slate. Its extensive thickness

(3100m) (Hobson, 1976; Dineley, 1966) must have required regional subsidence.

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-5

The Dartmouth Slate is comparable to the Lower Old Red Sandstone of Wales (King, 1934;

Allen, 1963, 1964, 1970, 1974; Allen & Tarlo, 1963; Tunbridge, 1981) and possibly represents

the first sediment deposited in Devon since Late Precambrian times. This view is based on the

claim by Cope & Bassett (1987) that prior to sedimentation of the Dartmouth Slate a

Precambrian landmass, Pretannia existed, which included Cornwall, Devon and the Bristol

Channel.

During the Early Palaeozoic, Pretannia in the south supplied sediment to the Welsh Basin in

the north. Caledonian uplift of the Welsh Basin, that formed St George's Land, caused a

reversal of the drainage (Cope & Bassett, 1987).

It is not supposed that the Devonian of South Devon and South Wales necessarily lay in the

same tectonic terrane. However there must have been a terrestrial pathway which allowed

fluviatile systems to inter-link at least during Pragian and Emsian times.

Fig. 3.2 summarises hypotheses (1) & (2) above. a(i) represents the Early Palaeozoic northerly

drainage from Pretannia into the Welsh Basin. a(ii) represents the Early Devonian reversal of

drainage following Caledonian uplift. Extension in South Wales and Devon is directly related.

b(i) represents the Early Palaeozoic northerly drainage over a hypothetically faulted basement.

b(ii) represents Early Devonian extension unrelated to extension in South Wales, causing the

local southerly drainage.

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Page 3-6

Although tectonic unrest is also signalled by the occurrence of Lower Devonian volcanics in

South Devon (Durrance, 1985), in support of hypothesis (3) above, it is possible that the most

significant effect on sedimentation was brought about by marine transgression. Devonian

eustatic sea level change may have had widespread influence due to the passivity of the

tectonic environment in which Pretannia was situated during Pragian and Emsian times,

replacing the fluvial environment with shallow marine conditions (Dineley, 1961; Simpson,

1951; Richter, 1967 and Hendriks, 1959). An acme of transgression could have resulted in

carbonate sedimentation in South Devon in Mid Devonian Eifelian times (House, 1975).

If there were little tectonism and no major structural break between South Wales and South

Devon, the North Devon area would be expected to be underlain by Lower Devonian strata

transitional in facies character to those of South Devon and South Wales.

Mid Devonian

There are three possible hypotheses for the Mid Devonian history of SW Britain: (1) areas in

SW Britain were uplifted due to regional End-Caledonian, Acadian tectonism or regional

strike-slip; (2) uplift was local and due to continued extension; and (3) a passive margin

setting persisted from the Early Devonian in which the distribution of Mid Devonian facies

was controlled by a eustatic rise in sea level.

Strata of Mid Devonian age are well represented in North Devon. In argument for hypothesis

(1), examination of Lower-Middle Devonian strata in South Wales and North Devon by

Tunbridge (1986) revealed evidence for a Mid-Devonian Bristol Channel Landmass and

postulated that it was generated by major strike-slip movement as a result of End Caledonian

tectonism. Evidence for the Bristol Channel Landmass includes the southerly derived Lower-

Middle Devonian conglomerates in South Wales eg the Ridgeway Conglomerate (Williams,

1971) and the Llanishen Conglomerate (Owen, 1974) and northerly derived sandstones in the

Hangman Grits (Tunbridge, 1980). Cope & Bassett (1987) suggested that the northern flank of

Pretannia could have been uplifted again to form the landmass so that tectonism in the Bristol

Channel area must have been at least substantial enough to involve Precambrian basement.

A recent structural model by Barnes & Andrews (1986) for South Cornwall suggests that

during Middle and Late Devonian times the tectonic setting of South Cornwall also involved

strike-slip movement, associated with the generation of oceanic crust and the deposition of

melanges and conglomerates containing exotic metamorphic clasts in a complex Gramscatho

basin.

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Page 3-7

Whilst it is difficult, owing to Variscan overprinting, to define the exact structural cause of

landmass or basin formation, it nevertheless remains clear from the work of Barnes &

Andrews (1986) that the tectonic environment had changed from the passive Early Devonian

setting.

In argument for hypothesis (2), North Devon was a sink for marginal shallow marine and

fluvial sedimentation (Tunbridge, 1983). A shift in fluvial sedimentation from South Wales to

North Devon probably marks an uplift event represented in South Wales by the Mid Devonian

unconformity (Owen, 1974) and in the Bristol Channel by the formation of the Bristol

Channel Landmass. Uplift in the north may have been due to local footwall uplift whilst

sedimentation in the south may have been accommodated by the extension.

An interesting feature in the Mid Devonian history of Devon is that as North Devon was being

uplifted, South Devon was being transgressed. This appears incongruous unless a rise in sea

level is considered (hypothesis 3) eg uplift in South Wales and the Bristol Channel may have

exceeded the rate of sea level rise so that only South Devon became submerged.

Further investigation into the type of fault control on the well established Devonian sequence

of North Devon is necessary; some structural reasons favouring a phase of Devonian

extension are given in Chapter 6.

Late Devonian-Dinantian

Fault controlled sedimentation occurred during Late Devonian times in South Wales (Powell,

1989) and South Devon (Selwood & Thomas, 1986; Selwood, 1990). Although not

documented, faulting probably also occurred in North Devon during these times. However,

investigation of Middle and Late Devonian sequences reveals no evidence for this type of

structural control, as discussed in section 3.2.3.

The transitional sequence across the Devonian-Carboniferous boundary at the base of the

Culm Basin succession illustrates the greatest regional effect on sedimentation during the Late

Devonian, namely, a northward transgression (Anderton et al, 1979) which established deep

water conditions to the south of the Bristol Channel and a carbonate platform to the north in

South Wales (Ramsbottom, 1973; George et al, 1976) by Early Carboniferous times. The

Bristol Channel Landmass of Mid Devonian origin must have been submerged during this

time since very little terrigenous material was supplied to South Wales and North Devon.

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Page 3-8

However in argument for fault activity, it is possible that Lower Carboniferous carbonates

deposited in the Mid Bristol Channel and Somerset area (Lees & Hennebert, 1982) developed

on a fault-related topographic high. It is also possible that the limestone area was separated

from the chert area of north Devon by a major fault line related to the structure that formed the

Devonian Bristol Channel Landmass. The major facies change across the Bristol Channel may

therefore reflect continued fault activity from Mid Devonian to Early Carboniferous times.

Late Carboniferous

A distinct change in structural regime is marked by the onset of turbiditic sedimentation in the

Culm basin and siliciclastic sedimentation in South Wales. These events mark the beginning of

Variscan thrusting (Kelling, 1988; Gayer & Jones, 1989) in the Bristol Channel Borderlands.

Prior to this Variscan episode one possibility is that extensional faults formed the landmasses in

Devonian times and subsequent facies boundaries during the Late Devonian and Early

Carboniferous. Such faults could have been reactivated as thrusts in the North Devon and

Bristol Channel area (Chapter 6). It is significant here that sedimentation in the Culm Basin

ended as southerly derived lithic sediment was being deposited in South Wales (Gayer & Jones,

1989). This would suggest the presence of a Late Carboniferous tectonic landmass in the Bristol

Channel which again separated South Wales and Devon and could have been rejuvenated as a

consequence of inversion tectonics. Furthermore this landmass possibly loaded the crust as part

of a composite load extending southwards into Devon and Cornwall to form the peripheral

foreland basin of South Wales in which further structures would have been reactivated.

3.2.3 EVIDENCE FOR SYNSEDIMENTARY TECTONIC DEFORMATION DURING THE

LATE DEVONIAN IN NORTH DEVON.

The sedimentology of the Upper Devonian rocks of the South Morte Bay coastal section were

examined for evidence of Late Devonian synsedimentary faulting, in the light of work in SW

Dyfed by Powell (1989) and in South Devon by Selwood & Thomas (1986) which

demonstrated that Upper Devonian sediments were deposited in fault controlled sub-basins.

Documentation of pre-Variscan extension in North Devon would be evidence for precursor

structures for positive inversion during Variscan deformation.

Evidence such as the presence of local coarse clastics or lateral facies variations was sought in

the coastal sections of North Devon.

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Page 3-9

The following stratigraphic section (Fig. 3.3) was measured through the Upper Devonian

Pickwell Down Sandstone (Edmonds et al, 1979) and the succeeding Upcott Slate of Goldring

(1971).

The section was measured in two parts: a lower incomplete section and an upper complete

section. For comparison with the upper section, isolated outcrops of Pickwell Down

Sandstone were measured from central parts of Morte Bay forming the lower incomplete

section. Details of the sections are given in Appendix 3.1.

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Page 3-10

In summary, the section provides no evidence of synsedimentary faulting; the lateral

continuity of the formations (BGS 1:50 000 scale sheets 292 & 277 Bideford & Lundy and

Ilfracombe) is evidence rather for tectonic quiescence. Good evidence is however preserved

for tidal and shallow marine sedimentation (Figs. 3.4a & 3.4b) which contrasts with the

interpretation of the sequence by Goldring (1971) as a back swamp alluvial or shallow

freshwater lake deposit; though Edmonds et al (1985) state that the Upcott Slate formed in

disturbed shallow waters of rivers, lakes, deltas, lagoons and seas.

The overall upward-fining succession could have been produced by progradation of lagoonal

and a muddy tidal flat and near shore marine sediments. Marine transgression could have

introduced the succeeding Baggy Sandstone (Goldring 1971) as a barrier sandstone sequence.

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Page 3-11

The Upper Devonian stratigraphy of North Devon reveals evidence which emphasises the

marine influence on sedimentation from the prodeltaic Morte Slate (Webby, 1966) through to

the Pilton Shale (Edmonds et al, 1985; Goldring, 1970) with the Upcott Slate possibly

representing the culmination in regression into tidally influenced waters.

3.3 CASE STUDIES OF FAULTS IN NORTH DEVON AND NORTH CORNWALL,

VARISCAN KINEMATICS AND STRUCTURAL STYLE.

Field work was carried out in the North Devon and the Culm basins to investigate the

movement history of faults as well as the general structural style of deformation. In particular

it was intended to explore the possibility that Variscan structures might (in some cases) result

from inversion of earlier extensional structures.

3.3.1 THRUST DEFORMATION ALONG THE NORTH DEVON COAST

Examination of Devonian strata along the North Devon Coast between Woolacombe and

Porlock (Fig. 3.5) has revealed two populations of Variscan faults, a set of conjugate strike-

slip faults and thrusts.

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Page 3-12

The thrusts show variable orientations and scale. The style of deformation is here thought to

be closely related to the lithological control imposed by the various Devonian formations and

also of possible significance to fault inversion history related to the phase of Variscan

deformation in which they were formed.

An example of lithological control on the style of deformation is the contrast in competence of

the relatively undeformed Baggy Sandstone and the Morte Slate. The Baggy Sandstone is only

affected by regional folding whereas the Morte Slate contains a pervasive cleavage, thrusts

and metre to decametre wavelength folds.

Two types of thrusts can be identified on the basis of their orientation. The first set are

mesoscale northward and southward transporting ramp thrusts which have been reoriented into

anomalous northward and southward dipping orientations. The second set of thrusts are

mesoscopic to regional in scale and transport Devonian rocks northwards with displacements

possibly exceeding 2km. In the Taunton area (BGS 1:50 000 scale sheet 295, Taunton),

Devonian rocks are thrust over Carboniferous Limestone and Millstone Grit (Whittaker, 1975)

whilst in North Devon decametre-scale thrusts and possible thrust-related folds of kilometre

scale are observed along coastal sections.

Example 1: Thrusts in Woody Bay and Lee Bay (SS680 490, SS691 494)

On the southern limb of the North Devon Anticline, (Fig. 3.1a & b) the Hangman Grit

Formation (Fig. 3.1a) contains decimetre to massive bedded bioturbated fine to medium

grained wacke which shows a well developed S-asymmetry cleavage and metre-scale in-

sequence thrusting (Fig. 3.6) with ESE strikes. The geometry of the thrusts changes from

thrust ramp to thrust flat, such as at Crock Pits (Fig. 3.7). Well developed thrust ramp

geometries occur in Lee Bay (Fig. 3.8). This is a typical geometry of thrusts within the upper

part of the Lynton Beds and Hangman Grit Formation and, possibly, of thrusts beneath the

Bristol Channel (see to Chapter 6).

Thrusts in Woody Bay and Lee Bay displace beds which have a regional moderate southerly dip

in the southern limb of the North Devon Anticline. The thrusts are not reoriented so are syn-

regional scale folding.

Example 2: Thrusts in Morte Bay, Woolacombe (SS454 438)

Detailed mapping of the Morte Slate (Fig. 3.1a) along the coastal section between

Woolacombe and Rockham Bay reveals decametre-scale folds (Fig. 3.9a).

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Page 3-13

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Page 3-14

Rapid changes in the dip of bedding and cleavage along the section and comparisons of the

large scale structure and minor structures suggest that these folds are thrust related (Fig. 3.9b).

The orientation and decametre scale of the folding are similar to those at Foreland Point

indicating that the folding and thrusting from Morte Bay to Bull Point and from Foreland

Point to the east are coeval. However the concentration of cleavage, minor folding and small

scale thrusting including back thrusting suggest that the incompetent nature of the Morte Slate

also controlled the style of deformation so that some minor structures may have formed by

accommodation.

Example 3: Thrusts at Ilfracombe, The Outfalls and Capstone Point (SS515 479, SS519 481)

The Kentisbury Slates of the Ilfracombe Beds at The Outfalls contain mesoscopic reoriented

south-dipping thrusts (Fig. 3.10). One such thrust contains a cleavage duplex which indicates a

southward direction of transport (Fig. 3.11).

The anomalous southward-directed thrust transport in relation to the southerly dip suggests

pre-regional fold back thrusting.

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Page 3-15

The southward directed transport points to an early thrusting phase rather than thrust

accommodation in the incompetent Kentisbury Slates during late folding (Fig. 3.12). Evidence

for late thrusting comes from mapping at Capstone Point which reveals decametre scale ramp

anticlines (Fig. 3.13a) and a variety of minor structures showing cross cutting relationships

(Fig. 3.13b).

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Page 3-16

Example 4: Thrusting at Heddon's Mouth (SS655 497)

ESE trending northward transporting thrusts and southward directed back thrusts at Heddon's

Mouth (Fig. 3.14) occur in the red stained upper sections of the Lynton Beds and the lower

sections of the Hangman Grit (Fig. 3.1a). Thrusting shows an anomalous dip in back

steepened beds on the southern limb of the North Devon Anticline. Therefore these thrusts

have been assigned a pre-regional fold age. Conversely the anomalous dip could have been

formed due to local stacking and reorientation in the hangingwall of a late thrust.

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Page 3-17

Example 5: Wild Pear Beach Fault at Hangman Point (SS580 477)

The steeply south-dipping fault downthrowing the Wild Pear Slates (Webby, 1965) in the

south against the Hangman Grit, displays an extensional geometry (BGS 1:50 000 scale sheet

277 Ilfracombe, SS582 479). Folding and possible thrusting at the base of Hangman Point

however display a distinct compressional geometry (Fig. 3.15).

This points to a long movement history beginning with the reactivation of major E-W and

ESE-WNW faults with the compressional structures being formed during this reactivation

event.

The present extensional geometry is probably due to Mesozoic extension (Chapter 7). This

points to two phases of inversion Devonian-Carboniferous positive inversion and

Carboniferous-Mesozoic negative inversion.

Example 6: (SS785 500, SS821 491, SS899 493, SS957 483)

Further examples of mesoscopic thrusts (some fold-related) occur to the east of Foreland

Point, eg, at Wingate, Culbone, Hurlstone Point, and Minehead (Figs. 3.16a & 3.16b).

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Page 3-18

In summary, the Devonian rocks of the North Devon basin contain a variety of Variscan

thrusts which in some cases may have Devonian, early Variscan or late Variscan origins.

Further investigation into the effects on shallow structure of positively inverting a basement

network of faults could reveal how early thrusting is replaced by regional scale thrusting.

3.3.2 CARBONIFEROUS SYNSEDIMENTARY FAULTING AND EARLY

EXTENSIONAL FAULTING IN NORTH DEVON.

Associated with the study of thrusting in North Devon, investigation was made into the pre-

thrust history. Evidence for extension pre-dating Variscan thrusting was discovered at Hartland

Point where an excellent example of extensional synsedimentary faulting occurs in the Bude

Formation (Fig. 3.1a). Further evidence for pre-thrust extension was also discovered at

Crackington Haven, Sandy Mouth and Upton Cross.

Two possibilities of fault history exist. The first involves net extension of the North Devon

and Culm Basins during the Carboniferous (i.e. an early extensional phase). The second

involves kinematically thrust-related extension (Mapeo & Andrews, 1991)

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Page 3-19

and, also, synsedimentary faulting (i.e. a composite deformation phase). Field evidence from

the following localities variously supports the former model of early extension and the latter

model of syn-thrusting extension.

Example 1: Hartland Point (SS230 277)

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Page 3-20

The prevalent structures at Hartland Point are decametre-scale chevron to tight rounded folds

and northward transporting folded thrusts (Fig. 3.17). A series of mesoscale listric extensional

synsedimentary faults contain sandstones in the hangingwalls which thicken towards the fault

planes (Fig. 3.18). These represent the best evidence for pre-Variscan thrust synsedimentary

fault movement. However, apart from other localities such as Upton Cross (Fig. 3.19; Enfield

et al, 1985), they appear to be a local phenomenon and do not represent a separate extensional

phase of deformation. Mesoscale thrusting has interfered with the synsedimentary faults, with

one showing evidence for reactivation (Fig. 3.20).

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Page 3-21

Example 2: Crackington Haven (SX143 968)

In the Crackington Formation, low angle mesoscale listric extensional faults on the NE side of

Crackington Haven have been reactivated as thrust faults and have been overridden by

decametre-scale folds above a sub-horizontal thrust (Fig. 3.21). It is unclear whether the

extension and compression were kinematically linked. Only one set of dip-slip slickenside

lineations was observed on the listric fault planes so there is no proof of a separate

reactivation phase apart from the metre scale coexistence of compressional folds and the

extensional displacements along the faults.

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Page 3-22

Example 3: Sandy Mouth (SS201 100)

Thrust related folds along the Sandy Mouth coastal section (Fig. 3.22) show evidence for a

composite thrusting history and, also, for an earlier extensional event. Figs. 3.23 & 3.24 show

the stratigraphy of hangingwall and footwall blocks and the kinematics along the various thrusts

in a decametre-scale structure. Correlation of beds across the fault planes shows that the

hangingwall block sequence is much thicker, pointing synsedimentary extensional faulting.

Figs. 3.24 & 3.25 show the similarly composite kinematics of the succeeding thrust phase.

3.3.3 COMPOSITE FOLD AND FAULT HISTORIES IN THE CULM BASIN.

The coastal section through the Culm Basin reveals that shortening was accommodated

mainly by chevron folding (see field sketches in Appendix 3.2) which, as in the Sandy Mouth

section could have been associated with a composite episode of thrusting and extension.

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Page 3-23

The Welcombe Mouth section is another example showing the composite nature of chevron

fold evolution and faulting.

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Page 3-24

Example: Welcombe Mouth (SS213 180)

The diamond shaped chevron folding at Welcombe Mouth displays a well-developed

horizontal cleavage fabric which may have been reoriented by a late flattening event (Fig.

3.26) or may have been formed later and overprinted an early steeply dipping cleavage

associated with the initial chevron folding. Late faults also affect the two axial planes of the

diamond shaped fold suggesting that post-fold faulting also occurred.

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Page 3-25

3.3.4 NET KINEMATIC REGIMES IN THE CULM BASIN

Field sketches of Appendix 3.2 show the fold-dominated nature of shortening in the Culm

Basin. It is interesting to speculate how a Devonoid faulted basement has affected Variscan

deformation. Evidence has been given to show that early N-S extension has occurred in the

Culm Basin so a model involving the early extension is a possible alternative to a net

compressional model with subsidiary extension involved in the main deformation history (eg

Bless et al, 1989). Bed length estimates of shortening reveal an average minimum value of

about 50% in the northern Culm Basin. Allowing for Mesozoic extension, eg at Portledge and

Speke's Mill Mouth, and for the Variscan shortening, a two-dimensional restoration along the

coastal section can be presented to show an early extension of about 5% (Figs. 3.27 - 3.32).

Figures 3.27 to 3.32: section restoration of the Variscan geology along the North Cornish and

Devon coast lines between Millook Haven in the south and Hartland Point in the north. 3.27:

Basic structural section. 3.28: Synoptic structure. 3.29: Tentative linked thrust system. 3.30:

Model of reactivation. 3.31: Reconstructed section of Pre-Permian structure. 3.32:

Hypothetical extensional system (Pre-Variscan or Permian?).

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Page 3-27

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The model involving early extension has been tested by tracing the sedimentological changes

above a prominent shale marker horizon within the Bude Formation, the Sandy Mouth Shale

(Fig. 3.33).

These changes shown in Fig. 3.34 occur across major NW trending faults. A much finer

sequence is found to the north at Stanbury Mouth than to the south at Sandy Mouth which

suggests either that facies changes were gradual or that hangingwall and footwall sequences

differ due to synsedimentary extensional faulting along the NW trending faults.

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Page 3-29

Further detailed stratigraphic surveying is required to enable a regional Variscan model to be

proposed. Nevertheless one possibility is that the initial response in the Culm Basin to thrust

loading was an extensional rearrangement involving the NW trending lineaments and the E-W

faulted margins of basement blocks. Possible early extension along NW lineaments and along

E-W trending faults such as the major Rusey Fault Zone, in the south, and decametre length

faults from Bideford to Instow, in the north, was succeeded by positive inversion. Inversion

could have formed the observed tightened southward asymmetric pop-up structure

encompassing the whole Culm Basin, and reactivated the NW trending lineaments as the

closely spaced set of dextral strike-slip faults.

3.3.5 ORDERS OF MAGNITUDE OF STRUCTURES

Edmonds & Freshney (1980, BGS 1:50 000 scale sheet 307 & 308, Bude) stated that numerous

periclinal folds, ranging in length from tens to hundreds of metres, with limb lengths up to

500m, are impressed upon the major flexures in the Culm Basin.

It is vital to identify the scale and spacing of the basement structures beneath the Culm Basin

that may have influenced any possible early extension, sedimentation and the observed late

thrusting and folding. Examination of BGS maps illustrating the spacing of the NW trending

faults throughout the Culm Basin shows that their spacing is similar to the wavelength of

'cliff-scale' chevron folds along the North Devon coastal section. The decametre scale folds

and closely spaced NW trending faults are defined here as 3rd order structures. Larger folds of

about 5.5km wavelength scale, eg the anticline at Embury Beach and also folding shown in

Fig. 3.36, are defined here as 2nd order structures.

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Edmonds & Freshney (1980) stated that the sense of overturning of folds is highly variable,

but may be related both to position within the structural belt and to collapse of minor folds

down the flanks of major folds. The fanning of the facing direction of folds in the Culm Basin

is considered by Ford (1990) to be due to hinterland escape along the top of a tilted fault block

(Fig. 3.35). This basin scale folding is defined here as first order.

In an inverted fault block model the origin and orientation of a number of second and third

order folds, in the overlying basin, is due first order fault reactivation. This would be

especially prevalent near the margins of the overlying basin. An investigation was therefore

made along the northern margin of the Culm Basin for evidence to indicate the occurrence of

a deep structure that affected deformation nearer to the surface.

The only lines of evidence for a sub-Culmian basement fault are (1) a change in fold facing

across the Abbotsham area unrelated to 2nd order folding (Fig. 3.37) and (2) a local zone of

E-W trending strike faults between Littleham (3.5km south of Bideford) and Instow. One of

the E-W strike faults at Gammaton Moor has an extensional displacement of at least 600m

and may have a substantial length and be basement related. It downthrows a thick sequence of

Bideford and Bude Formation, in the south, against a thinner sequence of Crackington and

Bude Formation, in the north (BGS 1:50 000 scale sheet 307/308, Bude). Further east, south

of Ilfracombe an extensional fault, with a downthrow of about 200m, juxtaposes

Carboniferous Chert and Crackington Formation to the south and Pilton Shale to the north.

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It is possible that the difference in thickness of Carboniferous sequence between the

hangingwall and footwall of the fault near Gammaton Moor, and generally between the

Bideford and the Hartland, is due to synsedimentary extension (as discussed previously). The

fault could have a long movement history.

3.3.6 LATE VARISCAN REACTIVATION OF EARLY VARISCAN FAULTS.

Evidence has been given to show that faulting in the Culm Basin was composite. The latest

phase of compressional Variscan deformation is represented by conjugate NE and NW

trending strike-slip and oblique-slip faults.

At Westward Ho! there is good evidence for a thrust fault that has been reoriented by folding

into a steeply dipping structure and reactivated as a strike-slip fault (Fig. 3.38). The latter

would represent the latest phase of Variscan fault reactivation in the Culm Basin.

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3.4 COMPARISON OF THE GEOLOGY OF DEVON AND CORNWALL AND THAT

OF SOUTH WALES: EVIDENCE FOR MAJOR STRUCTURES UNDER THE

BRISTOL CHANNEL.

There are major differences of Upper Palaeozoic stratigraphy to the north and south of the

Bristol Channel (Fig. 3.39). The Variscan structures beneath the Bristol Channel therefore

separate two different stratigraphic provinces (Gardiner & Sheridan, 1981). In contrast seismic

data from the Vale of Glamorgan (Chapter 5) and structures within the opencast coal sites of

South Wales (Chapter 4) indicate that north directed thrusting extends across the Bristol

Channel at least to the north crop of the South Wales coalfield. However the intensity and style

of deformation is generally different (as shown in later examples).

Fig. 3.39 illustrates the general Upper Palaeozoic stratigraphy of Devon and Cornwall in

comparison to that of South Wales. Tunbridge (1986) suggested that the Mid Devonian

Bristol Channel Landmass formed along a major fault within the Bristol Channel area.

Therefore there is stratigraphic evidence for the presence of at least one influential fault

offshore. Seismic investigations in Chapter 6 illustrate a candidate reflection event for this

fault.

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The structural variation across the Bristol Channel also points to the presence of a major fault

offshore. A comparison of the geometry of the major reflection event and the mesoscale

structure in the Bristol Channel Borderlands suggests it represents a major Variscan thrust

(Chapter 6). It is possible that either the fault responsible for the Devonian Bristol Channel

Landmass was partly reactivated as a thrust, during Late Variscan deformation, to form a

rejuvenated landmass, or, a further structure exists beneath the Bristol Channel as part of a

basement related fault zone. Chapter 6 illustrates the best seismic sections chosen to display a

composite fault zone with potentially a compound movement history.

The following examples of mesoscale structure emphasise the stronger deformation in the

Devonian rocks in the hangingwall (North Devon) of the offshore fault zone in contrast to the

gently folded Devonian strata in the footwall (South Wales).

Example 1: Foreland Point Section (SS750 510)

The fine-grained Lynton Beds (Fig. 3.1a) display a prominent pervasive cleavage which

strongly deforms bioturbation and centimetre-scale sedimentary structures (Fig. 3.40).

Measurement of bed length shortening due to folding and thrusting across the section suggests

a value of about 55%. This is clearly a minimum value not taking account of the effects of

cleavage. The estimated shortening value exceeds that for sections within the Culm Basin to

the south (50% this study and Hartley & Warr, 1990) and greatly exceeds that in the

Devonian of South Wales and the Welsh Borderlands, apart from areas around Llanstephan

(Chapter 4). A general N-S section at a scale of 1:250 000 through North Devon and the

southern part of the Inner Bristol Channel, however, reveals a shortening of about 20%. This

suggests that shortening estimates from Foreland Point may overemphasise complex

deformation close to a major thrust (the North Devon Coastal Thrust, Chapter 6). Evidence for

a lower regional shortening is the lack of deformation in Hangman Grits nearby, eg

immediately south of Lynton.

Example 2: Ilfracombe coastal section (SS524 479)

Cleavage development and small-scale tension gash structures in the Ilfracombe Slates

indicate a compressional and shear history associated with the formation of decametre

structures such as at Capstone Point (Fig. 3.13b). The mesoscale structures are discordant

suggesting that the compression was composite.

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No such development of cleavage and tension gashes is present in the Devonian strata of

South Wales apart from areas close to Caledonoid trending faults, eg, the Swansea Valley and

Carreg Cennen Disturbances. Cleavage development is far more intense in the Ilfracombe area

than in South Wales. This points to a strongly deformed hangingwall and less deformed

footwall sequence of the Bristol Channel Fault Zone (Chapter 6).

In terms of the offshore structure a similar intensity of deformation is expected at depth

beneath the Bristol Channel in the hangingwall of the Bristol Channel Thrust. The footwall

probably contains large-scale Variscan folds but a lack of small scale deformation.

Stratigraphically the Gravel Margin Thrust (Chapter 6) may mark a boundary between the

marine Devonian and Old Red Sandstone of South Wales, the Devon Brigantian Cherts and

Dinantian Limestone of South Wales and the turbidites of the Culm Basin and the Coal

Measures of South Wales.

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Seismic investigation of the deep structure of the North Devon by Brooks, Hillier &

Miliorizos (1993) also suggests a change in basement type and depth to basement across the

Bristol Channel Thrust.

All the above points indicate a major Variscan fault zone occurs beneath the Bristol Channel.

3.5 THE VARISCAN TECTONIC LOAD

Chapter 2 outlined the hypothesis that a load in the Variscan hinterland down flexed the

lithosphere to form the South Wales foreland basin. Examination of the North Devon area

reveals another Upper Palaeozoic basin which contrasts in terms of stratigraphy and structure

with the basin of South Wales. It is possible that the major structures beneath the Bristol

Channel represent the front of this load (Gayer & Jones, 1989; Hartley, 1991) which is now

concealed by the Mesozoic Bristol Channel Syncline (Evans & Thompson 1979).

Alternatively, the basins of North Devon may represent part of a composite load resulting

from the northward propagation of thrusting.

From the Devonian history of the Bristol Channel Borderlands it can be suggested that

basement was at shallow depths up to Mid Devonian times. The Variscan load may therefore

have been formed by basement which became allochthonous during Late Variscan times. The

lateral extent of the load, judged from the lateral extent of the foreland coal basins and the

extent of the Bristol Channel Fault Zone, would be a minimum of 200km. If the load was

composite, a north-south normal distance of a similar value is estimated. However the

occurrence of major transecting NW-SE trending cross faults eg the Sticklepath Fault and the

Cothelstone Fault (Chapter 6) and, also, the limited lateral extent of the Gravel Margin Thrust

may point to either a segmented Variscan load or a load of lateral extent limited to the present

study area.

3.6 NORTH-WEST AND NORTH-EAST TRENDING 'CROSS' FAUL TS OF NORTH

DEVON AND NORTH CORNWALL.

Measurement of the orientation of late faults which transect the Culm Basin and North Devon

Basin, here termed cross faults, points to a bimodal set of faults trending NE-SW and NW-SE

(Fig. 3.41). The movement sense tends to be oblique-slip to strike-slip, with the NW trending

faults being dextral and NE trending faults being sinistral, indicating N-S compression. These

faults tend to displace the chevron folds of the Culm Basin and the thrusts along the Devonian

North Devon coastal section and have been assigned to a late compressional strike-slip event.

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The Sticklepath Fault and Cothelstone Fault (Holloway & Chadwick, 1986; Chapter 6)

involve major dextral displacement on a kilometre scale.

Offshore seismic data from the Bristol Channel (Chapter 6) suggests that the cross faults

compartmentalised the Variscan structure, for example separating thrusts at Lower Palaeozoic

levels to the east beneath the Vale of Glamorgan (Chapter 5) from limestone-related thrusts in

the west. It is possible that prior to main Variscan compression a basement related system was

present during the earlier reactivation event. Cross faults could have developed from eg

Charnoid lineaments of Read & Watson (1975).

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Page 3-37

3.7 SUMMARY

The geological survey of North Devon has revealed a composite fault history extending from

the Mid Devonian to Late Variscan, as well as later Mesozoic extension (see Chapter 7). Case

studies have shown that the Variscan faults have composite histories even though they

probably developed within a consistently compressional regime. However, further work is

required near the Bideford area to investigate the role of extension in the distribution of the

Carboniferous formations. The main problem that arises from the deformation style in North

Devon is whether near surface structure was controlled by deep basement fault movement as

well as by later structural overprinting during the main compression.

3.8 ANALYSIS OF REGIONAL VARISCAN MODELS IN THE LIG HT OF THE

RESULTS FROM FAULT SURVEYS IN NORTH DEVON.

Evidence was provided by Badham (1982) for major strike-slip displacement during the

Variscan orogeny. A more recent model based on SWAT seismic data by Le Gall (1990,

1991) has emphasised the role of ramp thrusting in the Variscan fold belt beneath the Celtic

Sea and Bristol Channel area. There is good evidence for mesoscale ramp thrusts along the

whole of the North Devon coastal section.

Le Gall (1991) suggested that major thrusts originated from lower crustal levels and may

represent reactivated extensional margins of Devonian basins. From the fault study in North

Devon the evidence for Devonian extension is minimal, so that the application of this seismic

model to North Devon is highly questionable. However studies of fault spacing and fold

facing in the Bideford area could reveal new evidence for basement related faulting. Presently

the Devonian to Carboniferous positive inversion event remains open to debate.

Le Gall (1991) also suggested that the shallow structure is dominated by detachment-related

folding and thrusting. It is difficult to envisage how Devonian extensional margins could have

been incorporated into a thin-skinned thrust system, unless the structural interpretations of

Selwood (1990) are considered applicable to North Devon.

Two structural events are recorded in North Devon: (1) initial folding and thrusting,

hypothetically related to reactivation, and (2) late folding, thrusting and strike-slip,

hypothetically related to the main phase of basin uplift involving structural overprinting.

These two events may represent the D1 and D2 phases observed in North Cornwall by Warr

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(1991). It is possible that reactivation was followed by detachment-related folding and

thrusting (Le Gall, op cit)

In agreement with Le Gall's model, Variscan thrusting extends north of the front defined by

Gardiner & Sheridan (1981) so that the South Wales coal basin represents part of, or is

intimately related to, the Variscan foreland rather than a separate intracratonic structural unit.

Le Gall inferred that Devonian rocks at depth in South Devon and Cornwall represent intensely

stacked previously thinned crust. To the north this crust buttressed against pre-existing

Devonoid faults.

Le Gall finally stated that Caledonoid basement was wrench faulted in the foreland footwall of

the frontal ramp. This view is in agreement with the reactivation phase model for the Bristol

Channel Borderlands in which NW, NE and ESE trending structures were kinematically

linked during N-S Variscan compression. It does, however, conflict with observations that NE

& NW faults transect earlier Devonoid lineaments and Variscan thrusts.

A critical point in Le Gall's conclusion about the Variscan structure of the Bristol Channel

Borderlands is that the geometry of reflection events suggests thrusting rather than strike-slip

faults in a transpressional system (Sanderson & Marchini, 1984). This raises the question of

the Variscan transportation direction. Evidence from thrusting and folding in the Culm Basin

suggest an N-S compression whilst in the North Devon Basin and Bristol Channel ESE

striking thrusts transported strata towards the NNE. North of the Bristol Channel, main thrust

transport is approximately due north in the lower and western section of the coalfield (Chapter

4). Seismic data and mesoscale structure therefore point to northward directed thrusting.

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Appendix 3.1 Upper Devonian Stratigraphy

Lower Incomplete Section (central Morte Bay):

Near the base of the Pickwell Down Sandstones there are

metre bedded centimetre colour banded ferric mafic lithic

quartz wackes which are medium to fine grained and

contain subangular to subrounded low sphericity quartz

grains signifying a submature texture. Passing up sequence

metre trough cross stratified and centimetre cosets are

within light grey to tea green fine grained quartz wacke

containing subangular, moderate sphericity grains.

Decimetre subsequences pass transitionally into dark tea

green silty wacke. Cross bedding indicates a southerly

directed sediment transport. Centimetre bands in the wacke

outline possible wave forms. These pass into slightly

banded grey brown and tea green decimetre cross bedded

silty very fine grained quartz wacke and fine grained lithic

quartz wacke. The silty wacke is centimetre bedded and

ripple cross laminated. There is an upward darkening in

colour of spotted grey sandstone.

In upper parts of the incomplete section, light grey and

tea green decimetre bedded centimetre laminated and

centimetre ripple bedded passing to centimetre planar

bedded calcareous quartz wackes show a range in grain

size from silty to very fine and fine grained sandstone

indicating a moderate sorting in a submature texture. The

lower section contains deep buff purple very fine grained

sandy, medium grained green chloritic silty quartz wacke

which is ferric rich and poorly sorted. Decimetre to

coarse centimetre bedding is characteristic in upward

fining subsequences of centimetre ripple cross laminated

to ripple laminated and parallel laminated siltstones.

In uppermost sections of the Pickwell Down Sandstone

there is an overall upward coarsening of sediment, with

more fine sandstone and less siltstone with an upward

increase in size and amount of ripple cross lamination.

Buff red and tea green siltstone continue until the upper

section with sigmoidal ripples observed to be asymmetric

towards the west (Fig. 3.4b).

Upper Complete Section (south headland of Morte Bay):

At the base of the upper section measured in Putsborough

Sand, South Morte Bay metre bedded and centimetre

banded medium grey and buff red very fine grained silty

ferric quartz wacke shows decimetre trough cross

stratification with centimetre cosets in upper parts of the

metre beds. Sedimentary structure then changes into

centimetre planar bedding showing flat planar bases

accompanied by an upward transition in petrological

composition from quartz wacke to fine grained

micaceous very fine grained to silty quartz wacke. Sharp

cut offs define the tops of these subsequences (Fig. 3.3a).

Following subsequences extend over 2m and consist of

upward fining units containing buff red and grey very

fine grained sandy micaceous quartz siltstone displaying

centimetre lamination and decimetre planar bedding.

4m of buff red and grey green ripple cross laminated

siltstones complete a larger metre scale subsequence

trending from a sandstone to siltstone dominated

grainsize. Within these siltstones sedimentary structure

changes from planar bedding to rippled bedding. On

closer inspection an upward decrease in grainsize on a

decimetre scale from siltstone with very fine grained

sandstone through to mudstone with siltstone is

accompanied by a change in sedimentary structure from

decimetre cross bedding to centimetre and millimetre

ripple cross lamination. Towards the top of the 4m

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sequence the proportion of sandstone increases, though

the subsidiary subsequences remain upward fining.

There then follows a 2m upward coarsening transition

from buff red sandy siltstone to tea green sandy siltstone

with silty wacke. The wacke contains low amplitude

(10cm) trough cross bedding with 3m wavelengths and

centimetre ripple cross lamination. Compositionally it

contains mica, lithics and quartz.

About 8m of decimetre upward fining subsequences

follow which contain three transitional constituent units.

At the base of each subsequence cross bedded sandy

siltstone passes to ripple laminated and parallel laminated

muddy siltstone. Further inspection shows that in mid

subsequence levels grey green and buff red silty wacke

and sandy siltstone pass into laminated muddy siltstone

and that in upper subsequence levels there is buff red

decimetre bedded sandy siltstone and muddy siltstone

(Fig. 3.3b).

A change in subsequence type occurs on passing up

sequence. The transition between the previous

subsequences and the new sequence is marked by a rapid

upward thinning in bedding and an accompanied decrease

in grainsize.

Planar decimetre bedded red sandy siltstone displaying

wavy tops to beds forms separate units from succeeding

centimetre ripple cross laminated muddy siltstone and

centimetre thicknesses of green shale at the top of each

subsequence.

About 20m up sequence from the base of the upper section

new subsequences begin. These consist of decimetre to

metre ripple bedded grey brown silty and very fine grained

quartz wacke. Centimetre cosets form the metre beds and

the megaripples are asymmetric towards the north.

Centimetre tops to the subsequences consist of tea green

parallel laminated and ripple laminated siltstone. There is a

marked absence of siltstone within the metre bedded

sections of the subsequences.

Subsequences that follow consist of decimetre planar

bedded, light grey very fine grained micaceous lithic quartz

wacke with planar tops and sharp cut offs occurring

between the wacke and succeeding centimetre layers of

light grey to tan brown silty claystone showing no

lamination (Fig. 3.3c).

Composite subsequences on a 5m scale follow and

consist of decimetre to centimetre bedded grey green

calcareous lithic quartz wacke with very fine to fine

grained centimetre wavy tops to the bedding and loaded

bases. The wacke passes upwards rapidly through a ripple

cross laminated siltstone into tea green silty claystone

near the top of the sandstone unit (Fig. 3.3d). Closer

inspection of middle to upper parts of the subsequence

reveals an increase in bedding thickness to a decimetre

scale and a transition of the fine grained sediment from

silty claystone to muddy siltstone and predominantly

ripple cross laminated siltstone. Following sandstone

units show centimetre trough cross stratification with

centimetre cosets and millimetre laminae showing colour

banding which pass into centimetre ripple cross

lamination (Fig. 3.3e). In uppermost sections of the 5m

thick composite subsequence decimetre trough cross

bedded intraformational conglomerate passes into an

orange brown wacke containing phosphatic debris which

may represent fish or pelletal remains. These are

succeeded by tea green ripple cross laminated sandy

siltstone (Fig. 3.3f).

CHAPTER THREE



Page 3-41

The succeeding new subsequence consists of thick

centimetre planar bedded tea green to light grey very fine

grained quartz wacke. The base of the subsequence

shows a sharp cut off. The top 60cm of the sequence

fines upward to silty wacke with centimetre ripple cross

lamination.

Similar subsequences follow with the addition of new

structures such as convoluted bedding, nodular bedding

and load structures near the base of the subsequences.

A prominent development of ripples is found in the

banded buff red, tea green part of the sequence about

45m from the base of the section. Bimodal current ripples

are well developed in this part of the section showing

straight crested and sigmoidal crested forms in a siltstone

dominated sequence of millimetre banded light and dark

buff red siltstone with less sandstone than in lower parts

of the section. This may represent the transition from the

Pickwell Down Sandstone to the Upcott Slate. Details of

the ripples show that their wavelength is 4cm and

amplitude is 0.5cm with straight ripple crests plunging

ESE and sigmoidal ripple crests plunging WSW (due to

Variscan folding). The stoss sides of these asymmetric

ripples face towards the north (Fig. 3.4a). Ripple

interference patterns occur of straight crested ripples

asymmetric to the north and sigmoidal ripples

asymmetric to the south. These are very suggestive of a

bimodal or tidal influence in sedimentation.

Appendix 3.2

Examples of chevron folds from the North Devon - North

Cornwall coastal section between Bude and Millook

Haven.

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sedimentary responses to the developing Variscan orogeny.

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FIGURE CAPTIONS

Fig. 3.1 a. Sketch map of the Culm Synclinorium showing the position of the North Devon Basin, the Culm basin (and the

Trevone Basin). The Lithostratigraphic columns show the approximate thickness of the major formations in the North Devon

and Culm Basins. Key to North Devon Basin stratigraphy: LQB, Little Quantock Beds; LB, Lynton Beds (Lynton Slate); HG,

Hangman Grit; IB, Ilfracombe Beds; MS, Morte Slate; PDS, Pickwell Down Sandstone; US, Upcott Slate; ByS, Baggy

Sandstone; PS, Pilton Shale. Key to Culm Basin stratigraphy: CoCh, Codden Chert; Cr, Crackington Formation; Bi, Bideford

Formation; Bu, Bude Formation. Structure: the sketch map also shows the trends of the North Devon Anticline north of

Exmoor and in the Quantock Hills and the Sticklepath fault zone transecting the Culm Basin.

3.1 b. Sketch map of the North Devon Basin. Locations: B, Bideford; I, Ilfracombe; L, Lynmouth; M, Minehead. Key: 1.

Lynton Beds; 2. Hangman Grit; 3. Ilfracombe Beds; 4. Morte Slate; 5. Pickwell Down Sandstone & Upcott Slates; 6. Baggy

Sandstone; 7. Pilton Shales; 8. Carboniferous; 9. Triassic & Jurassic. (After Webby, 1965 and Edmonds et al, 1975).

Fig. 3.2 a. (i) The early Palaeozoic north-directed flow from Pretannia into South Wales. (ii) The reversal of drainage due to

Caledonian uplift. 3.2 b. (i) North-directed flow over a segmented basement. (ii) South-directed flows in South Devon during

the Early Devonian unrelated to the Caledonian uplift in South Wales.

Fig. 3.4 a. (i) Various current directions derived from the lee-stoss facing geometry of asymmetric current ripples. Key:

RCA, ripple crest axis; σ, sigmoidal ripple crest; l, linear ripple crest; B, bedding plane for restoration; CD, current direction.

(ii) Comparison of current directions obtained from a cross bed (Xb) and a sigmoidal ripple crest.

3.4 b. Interference patterns of bimodal ripples in fine grained siltstone of the Upcott Slates.

Fig. 3.5 Sketch map of the North Devon Basin showing the location of the survey areas described in the case studies. The

major cross faults and anticlinal fold axial traces along the North Devon coast are also presented. Stereographic projections (i),

(ii) & (iii) summarise the structure along this coastal section.

Fig. 3.6 Mesoscale in-sequence thrusts in the lower part of the Hangman Grit Formation at Woody Bay. Key: spotted beds

represent thick sandstone units.

Fig. 3.7 Change from thrust ramp to thrust flat geometry in the Lynton Beds at Crock Pits. Beneath the thrust flat there is a

development of tension gashes. The lower sandstone unit indicates a decimetre-scale displacement along the thrust ramp.

Fig. 3.8 Thrust ramps in the upper part of the Lynton Beds at Lee Bay showing the typical geometry of thrusts found also in

the Hangman Grits of Foreland Point. The hangingwall anticlines contain a non-pervasive cleavage. The footwall to a

particular thrust ramp (immediately above another ramp) contains a minor northward verging duplex.

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Fig. 3.9 a. (i) Map of the Morte Slate between Woolacombe and Flat Point. ESE trending thrusts, bedding and cleavage

constitute the structure. nb Dips inland are from BGS 1:50 000 scale sheet 292, Bideford and Lundy. Key: spotted ornament,

Pickwell Down Sandstone; wavy ornament, Ilfracombe Beds; plain ornament, Morte Slate. (ii) Stereographic projections of

bedding and fault π poles from the Morte Slate and a rose diagram of the cross faults transecting the coastal section indicating a

dominant NW trend. 3.9 b. Section from Barricane Beach (north of Woolacombe) illustrating the thrust related folding in the

Morte Slate.

Fig. 3.10 A pair of mesoscopic south-dipping and south-verging ramp thrusts within the Kentisbury Slates of the Ilfracombe

Beds at The Outfalls near Ilfracombe. The thrusts probably represent early back thrusts reoriented by late folding.

Fig. 3.11 Cleavage duplex along an early, reoriented back thrust showing good evidence for south-directed transportation

prior to folding.

Fig. 3.12 Schematic diagram illustrating the differences in facing of a back thrust deformed by late folding (a & b.) and

thrusts formed by accommodation and fold tightening (c. & d.).

Fig. 3.13 a. A cross section (AB) and plan of part of the wave cut platform at Capstone Point illustrating a late ramp anticline

and north-verging thrust in the Combe Martin Slates of the Ilfracombe Beds. 3.13 b. Minor structures (minor folding,

cleavage, tension gashes) in the Ilfracombe Beds showing cross-cutting relationships. The figures above illustrate that the

structural of the Ilfracombe Beds is composite. 3.13 c. The stereographic projection shows the relationship between cleavage,

tension gashes and bedding.

Fig. 3.14 a. Southward, back-tilted thrusts at Heddon's Mouth, near the boundary between the Lynton Beds and the Hangman

Grits. 3.14 b. The stereographic projection summarises the structure near Heddon's Mouth.

Fig. 3.15 a. A view looking east at Hangman point and the faulted boundary between the Wild Pear Slates and Hangman

Grits. Folding is evident at the base of the Headland. 3.15 b. The stereographic projection summarises the structure around

Hangman Point.

Fig. 3.16 a. Examples of faulting east of Foreland Point. Glenthorne Beach: there are complex folds associated with north-

verging thrusts. Yellowstone: a particular hangingwall anticline contains a pervasive cleavage with an anomalous asymmetry in

relation to a north-verging thrust indicated by the hangingwall fold geometry. Ivystone: possible low-angle cross fault or early

thrust. Hurlstone Point (near the Timberscombe Fault): open, north-facing chevron folds. Minehead: north-facing folding (west

of Minehead harbour). 3.16 b. The Stereographic projection summarises the structure between Wingate (east of Foreland

Point) and Porlock.

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Fig. 3.17 Tight rounded folds and folded north verging thrusts in the Bude Formation at Hartland point. The diagram also

shows the location of synsedimentary extensional gravity slides in relation to the folding. A thrust duplex is illustrated

immediately to the south of the gravity slides which incorporated in the thrusting the upper rotated segment of one of the

extensional faults.

Fig. 3.18 The synsedimentary gravity slides in the Bude Formation at Hartland Point showing the thickening of sandstone

units, in the hangingwall, towards the extensional fault planes. (Growth Faults).

Fig. 3.19 Early extensional faults, in the Bude Formation at Upton Cross, associated with the Black Rock Slump Bed

(BRSB). Late folding deformed the slump bed and probably reactivated the extensional fault planes.

Fig. 3.20 The reactivation of the rotated early extensional fault plane within the thrust duplex at Hartland Point, Bude

Formation.

Fig. 3.21 Structure in Crackington Formation showing the superposition of thrusting and chevron folding on an early

decametre-scale listric fault template.

Fig. 3.22 Sketch section of the chevron folding in the Bude Formation near Sandy Mouth. (The arrow indicating the location

of Sandy Mouth also locates the position of the structure in Figs. 3.23 & 3.24).

Fig. 3.23 Sketch section of a reactivated decametre-scale fault zone in the Bude Formation at Sandy Mouth. The lower part

of this section displays a thicker hangingwall sequence indicating early synsedimentary extension. The upper part of the

section displays a distinct reverse displacement associated with folding in the hangingwall.

Fig. 3.24 Sketch section showing the kinematics of the thrusting and folding events along the fault zone. Repetition of the

sequence, between bed (a) - highlighted with finely spotted ornament) and bed (b) - highlighted with speckled grey

ornament, in the hangingwall, contrasts with the smaller displacement, by the fault, of the main sandstone unit (c) above.

This may point to an oblique section through the structure.

Fig. 3.25 Sketch section of a pop-up structure in the Bude Formation at Sandy Mouth illustrating a composite compressional

history for faults a-d. Details of the fault planes illustrate early and late duplexes pointing to various directions of movement

prior and during folding.

Fig. 3.26 Sketch section of chevron folding (including 'diamond' folding) at Welcombe Mouth in Crackington Formation.

The diagram illustrates the composite nature of folding in the Culm Basin. Various cleavage fabrics developed close to the

hinge of a particular diamond fold which points to refolding or flattening of a tightened chevron fold probably by later shear.

Faulting along the axial planes of the folds emphasises further the composite nature of deformation.

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Fig. 3.33 Location of the Sandy Mouth Shale (SMS) in the stratigraphy of the Bude Formation. Key: HQS, Hartland Quay

Shale; SPS, Saturday's Pit Shale; WGS, Warren Gutter Shale.

Fig. 3.34 Comparison of the coarseness of the stratigraphic sequence immediately above the Sandy Mouth Shale, at Sandy

Mouth and Stanbury Mouth. The pre-deformational horizontal distance (restored length) between the sections at Sandy

Mouth is about 100m. The present distance between these sections and Stanbury Mouth is 3.5km. (Prior to folding it is about

7km). The measurement of coarseness was calibrated against a detailed stratigraphic section along the Sandy Mouth section

where (1) represents a shale sequence; (2a) represents a sequence of siltstone and very thin bands of sandstone; (2b)

represents a sequence of thin siltstone beds and decimetre sandstone beds and (3-4) represents a sandstone dominated

sequence.

Fig. 3.35 Cartoon of the Culm Basin relating the facing of folds to reactivation above extensional blocks. In the case of the

Culm Basin folds escaped towards the south above a gently dipping basement topography. (Fold axial planes are steeply

dipping due to the buttressing effect of a steep northern basin boundary expected in the Bideford area.

Fig. 3.36 A section through the Bude Formation from Higher Sharpnose to Sandy Mouth illustrating the wavelength of the

second order folds in the Culm Basin. (From Freshney et al, 1979; 1:50 000 scale sheet 307/308 Bude).

Fig. 3.37 Sketch section of the structure between Abbotsham and Greencliff in the Bideford Formation marking an area of

gradual change in fold facing ascribed either to the modifying effects of a buried basement fault or to the fanning across a

first order fold.

Fig. 3.38 Schematic structural history of the reactivation of the pre-fold thrust as a post-fold strike-slip fault (after

reorientation due to folding). This history is based on the structure of a cross fault found in the wave cut platform at

Westward Ho!

Fig. 3.39 Chronostratigraphic diagram showing the variation in facies across the Bristol Channel Borderlands in a. the

Devonian and b. the Carboniferous. Key: plain ornament, Hiatus; 1, shallow siliciclastic marine; 2, chert, starved basin; 3,

carbonate platform; 4, fluvial and marine (mainly marine); 5, fluvial and marine (mainly fluvial); 6, Braided fluvial.

Fig. 3.40 Compression related microstructures in the Lynton Beds at Foreland point. A strongly pervasive cleavage deforms

sedimentary features pointing to a significant amount of shortening in addition to that caused by folding and thrusting.

Fig. 3.41 Stereographic projection summarising the orientation of cross faults in the North Devon and Culm Basins. NW and

NE trending sets are evident.

Marios Miliorizos 7th June 2006 File name: PhD Chapter 3 Three

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