The Bristol-Mendip foreland thrust belt - Department of Earth Sciences

Journal of the Geological Society, London, Vol. 143, 1986, pp. 63-73, 9 figs. Printed in Northern Ireland
The Bristol-Mendip foreland thrust belt
G. D . WILLIAMS & T. J . C H A P M A N ’
Department of Geology, University College, PO Box 78, Cardiff CF1 ZXL, U.K.
Department of Geological Sciences, Plymouth Polytechnic, Drake Circus, Plymouth PL4 8AA, U. K.
Abstract: The thrusts and associated fold structures of the Bristol-Mendip area are interpreted in
terms of a thin skinned, piggy-back foreland thrust belt of Variscan age with dominant northward
transport. Strike-normal balanced sections and compatible strike-parallel sections have been constructed
to illustrate the 3-D geometry of the linked thrust system. North-south shortening of c.
20 km (40%) is calculated from deformed and restored strike-normal sections. A thick and extensive
thrust wedge of pre-Old Red Sandstone basement in the west of the area is responsible for
culminations expressed at the surface as the Mendip periclines. Uplifted culminations permit erosion
to lower stratigraphic levels in the west and south of the area. Strike-parallel sections and a hanging
wall cut-off map indicate that the majority of thrusts in the area are mainly oblique ramps rather than
simple frontal and lateral ramps, or flats. The en-echelon pattern of the Mendip periclines may be
related to the oblique NE climb of two of these thrusts. The angle of frontal ramp climb through the
Dinantian stratigraphy is generally low, and this explains the presence of numerous klippen of Lower
on Upper Carboniferous rocks, whilst suggesting that certain thrust sheets were much more extensive
than present outcrops indicate. Early, pre-thrusting movement on basement lineaments in the
Variscan foreland created uplifts, angular unconformities and a predominance
the north of the area.
of N-S structures in
The area studied here (Fig. 1) includes the Bristol and
Somerset coalfields and the Mendip Hills and it extends
from Wells in the south to Tortworth in the north in the
counties of Avon and Somerset. This area is covered by the
BGS special 1 : 63,360 scale map of the Bristol district, the
Sheets 280, Wells and 281, Frome at 1 : 63,360 scale, and the
1 : 50,000 Weston-Super-Mare sheet. The rocks of the area
include Cambrian and Silurian sediments at Tortworth;
?Silurian andesites and tuffs in the Beacon Hill area;
Devonian of Old Red Sandstone (ORS) facies; and Lower
and Upper Carboniferous rocks: all of which are involved in
Variscan deformation. Permo-Triassic and younger rocks lie
unconformably above this Variscan ‘basement’.
A remarkably perceptive account of the stratigraphy and
structure of the Mendips was given by De la Beche (1846) in
Geological a Survey map and memoir. The general
resemblance between Permo-Carboniferous structures in the
Bristol/Somerset coalfield and those of N France and
western England was noted in the mid 19th century
(Godwin-Austen 1856) and the possibility of intervening,
concealed coalfields was proposed. Other notable, early
contributions to the geology of the Mendips were made by
Bristow & Woodward (1871) on the coal prospects south of
the Mendips, and Woodward (1876) in a memoir on the
East Somerset and Bristol coalfields. Vaughan (1905)
introduced a scheme, using some Mendip localities, for the
subdivision of the Lower Carboniferous on palaeontological
grounds and both Sibly (1907) and Reynolds & Vaughan
(1911) applied this scheme to the rocks of the Mendips.
The structural complexities of the Mendips began to be
understood as early as 1824 when Buckland & Conybeare
described overfolded and greatly contorted Coal Measures
in the east Mendips. The faulted ‘inliers’ of Carboniferous
Limestone in the east Mendips were the source of much
debate (Woodward 1876; McMurtrie 1881; Winwood 1891;
Ussher 1889). These ‘inliers’ were considered to be either
autochthonous, block-faulted fragments or allochthonous,
63
-l
BATH
Fig. 1. Map showing the Bristol-Mendip area and the location of
the structural map (Fig. 2). AV, Avonmouth; CH, Cheddar; CL,
Clevedon; PH, Portishead; RS, Radstock; SH, Shepton Mallet;
TW, Tortworth; WL, Wells; WM, Weston-super-Mare.
overthrust pieces of Lower Carboniferous rocks, completely
surrounded by Upper Carboniferous rocks. Sibly (1912)
successfully interpreted the Vobster ‘inlier’ (see Fig. 2) as a
klippe due large to
scale overthrusting of Lower
Carboniferous above Upper Carboniferous rocks. Reynolds
& Greenly (1924) and Welch (1928, 1932) adopted the
overthrusting suggestion in interpreting the structures of
other parts of the Mendips.

Fig. 2. Structural/lithostratigraphical map of the Bristol-Mendip area showing section lines A, B, C and D; major thrusts T1 to
T3; and thrust zones T4 to T6. Klippen of Lower on Upper Carboniferous are labelled CK, Churchill; GK, Gordano; HK,
Harptree; VK, Vobster and Upper Vobster. Mendip periclines are labelled BHP, Beacon Hill; BDP, Black Down; NHP, North
Hill; PHP, Pen Hill. The ornament used for the various lithostratigraphic units is the same as on the cross-sections (Figs 6 , 7 and
8).

More recent work has culminated in the publication of
BGS maps and associated memoirs (Green & Welch 1965;
Whittaker & Green 1983). The crustal structure to the east
of the Mendips, based on deep seismic reflection profiles,
has been elucidated by Chadwick et al. (1983) and
re-evaluated by Williams & Brooks (1985). General
accounts of the geology of the Bristol-Mendip area have
been published by Kellaway & Welch (1948), Cave
and Kellaway & Hancock (1983).
(1963)
In this paper, we use as a basis the abundance of
published structural information in the form of maps,
cross-sections and mine and borehole data from the Mendip
area. These, coupled with our own field observations, have
enabled us to carry out detailed studies of individual thrust
faults and to elucidate the geometry of the Mendip thrust
system using balanced cross-sections. We present here a
thin-skinned interpretation for the Mendip foreland thrust
belt and we discuss the evidence for such a geometry.
Summary of stratigraphy
The oldest exposed rocks in the Bristol-Mendip area are the
Tremadocian shales of the Tortworth inlier (Curtis 1955)
(Fig. 1). Also Tortworth, at mudstones, calcareous
sandstones and limestones of Llandoverian and Wenlockian
age are present. Presumed Silurian volcanic rocks occur in
the core of the Beacon Hill pericline in the south of the
Mendips (Fig. 2 ) . The marine Silurian is overlain by a
E
0
v)
4 UPPER
1
d' I
BELT BRISTOL-MENDIP
THRUST FORELAND
65
COAL SERIES
PENNANT SERIES
QUARTZITIC SANDSTONE GROUP
HOTWELLS LIMESTONE
CLIFTON DOWN LIMESTONE
BURRINGTON OOLITE
I BLACK
---I
1
7
ROCK LIMESTONE
LOWER LIMESTONE SHALE
600 m-thick conformable sequence of Lower Devonian, Old
Red Sandstone continental facies which is unconformably
overlain by an Upper Devonian, Old Red Sandstone
sequence varying in thickness from 500 m in the south to
150 m in the north (Kellaway & Hancock 1983).
Several significant non-sequences are present in the
Dinantian succession of the Mendips (Ramsbottom &
Mitchell 1980), but the lithostratigraphy, based particularly
on oolitic marker beds, can be correlated successfully from
the Mendips in the south to the Avon Gorge in the north
(Whittaker & Green 1983). The total Dinantian sequence is
approximately 800 m thick in the Bristol Coalfield (Kellaway
& Hancock 1983). We have adopted the lithostratigraphic
scheme of nomenclature involving Carboniferous units d' to
d6 (Fig. 3) as portrayed on the Bristol District special sheet
(1:63,360). This scheme, although outdated, proved to be
more workable when cross referring fault cut-offs of
stratigraphy, from maps to cross section, than either the
biostratigraphy of Ramsbottom & Mitchell (1980) or the
detailed lithostratigraphy of Whittaker & Green (1983). The
Dinantian is represented by various facies of Carboniferous
Limestone deposited as platform or shelf type limestones (d'
to d3 of the present scheme). In the northern part of the
district, deposition was not continuous throughout the
Carboniferous and it is probable that uplift and significant
erosion took place along the NE-SW trending Lower
Severn Axis in mid-Carboniferous times, prior to the
deposition of the Upper Coal Measures (Kellaway & Welch
1948; Whittaker & Green 1983). Evidence of this is the
UPPER COAL MEASURES
L. a M. COAL MEASURES
MILLSTONE GRIT
CARBONIFEROUS LIMESTONE DINANTIAN
OLD SANDSTONE RED DEVONIAN
WESTPHALIAN
NAMURIAN-
Fig. 3. Lithostratigraphic summary and nomenclature of rock units involved in the Variscan
deformation.

66 G . D . W I L L I A M S & T. J . CHAPMAN
major overstep of Upper Coal Measures (d6) on to Old Red
Sandstone and Dinantian strata (Nat. Grid ref. 483734).
Namurian and Westphalian (d4, d5 and d‘) rocks of the
Bristol Coalfield attain a maximum thickness of 1900 m.
Major Variscan deformation occurred towards the end of
the Carboniferous giving rise to northerly-verging, asymmetric
folds and thrust faults. As Permo-Triassic and
younger sediments serve only to blanket the Variscan
terrane, they have not been studied nor are they shown on
our maps and cross-sections.
Structural synthesis
The geology of the Mendip-Bristol area is dominated by the
Variscan fold and thrust structures exposed through the
Permian-Mesozoic cover (Fig. 2). Old Red Sandstone and
older rocks are exposed in the extreme north of the area
(including Silurian rocks of the Tortworth inlier), and at
Portishead mainly in the core of an ENE-WSW trending
anticline. the south, In notable outcrops of pre-
Carboniferous rocks occur in the en-echelon, E-W trending
Mendip periclines named from NW to SE the Blackdown,
North Hill, Pen Hill and Beacon Hill periclines. Although
periclinal in general form, these folds are asymmetric and
northerly verging in N-S profile with steep, southerlydipping
axial surfaces. The E-W trend of structures in the
south of the area changes to a NE-SW or N-S trend in the
north.
Thrust faults have been recognized for many years in the
area and have been assigned a plethora of local names. We
propose that many of these separately named thrusts are
parts of single, laterally persistent structures. The isolated
klippen of Carboniferous Limestone indicate large thrust
displacements and this supports our interpretation of large
strike lengths for individual thrusts (Elliott 1976). We have
assigned the symbols T, to T, to major thrusts of the area:
the subscripts increase in the order of piggy-back thrust
development from hinterland to foreland. Thrusts T1, T,
and probably T, are single laterally persistent structures,
whereas T, to Ts are zones comprising several thrusts, which
may join or branch, or may be separate with overlapping
tips (see western part of T, on Fig. 2).
Thrust T, (see Fig. 2) is a large E-W thrust passing close
to Wells and Shepton Mallet. The klippen of Vobster,
Upper Vobster and Luckington are underlain by thrust T,.
Thrust T, stretches from Weston-Super-Mare in the west to
the Radstock Coalfield in the east and it underlies the
Churchill and Harptree klippen (local names: Bleadon
Thrust, South-Western Overthrust, Emborough Thrust and
Southern Overthrust). Thrust T3 corresponds the to
Farmborough fault belt (Green & Welch 1965). Farther
north (Fig. 2) a zone of thrusts, T,, extends from Clevedon
on the coast to NE of Bristol. Local names include
Clevedon, Naish House and Tickenham faults and this
structure continues into the Kingswood Anticline east of
Bristol. Kellaway & Hancock (1983) termed this zone the
Avon Thrust. The Gordano klippe is underlain by thrust T,.
Thrust T, is a concealed NNE-SSW splay from T, and
thrust T, is the NE-SW trending Ridgeway Thrust. Because
of their orientation, it is likely that thrusts T5 and T, have
undergone significant oblique-slip movement during Variscan
shortening.
The existence of various klippen related to three of the
major thrusts helps to elucidate the 3-D geometry and
tectonic history of the area. A strike-normal section across
the Harptree klippe and North Hill Pericline (Fig. 3) shows
steeply dipping to inverted Carboniferous Limestone (d’)
thrust above Coal Measures (d’) on thrust T,. In a
piggy-back thrusting interpretation, this thrust has been
folded above the North Hill Pericline, now eroded, but is
preserved to the north in the core of a syncline. The
evolution of this structure (Fig. 4) involves movement on an
early thrust (TJ which puts Dinantian on Westphalian
strata. A subsequent, lower thrust creates the North Hill
pericline as a tip line fold (Williams & Chapman 1983;
Hossack 1983) responsible for folding of the overlying strata
and the overlying thrust T,. It is clearly seen on both the
Bristol and Wells 1:63,360 sheets that thrust T2, labelled
‘South-Western Overthrust’ and ‘Emborough Thrust’ on
those maps, is folded around the North Hill Pericline (Fig.
2). This relationship, and those of other klippen,
demonstrates the piggy-back nature of the thrust evolution.
Lower and more northerly thrusts
later than the folded upper thrusts.
are clezrly developed
Blind thrusts with frontal tips beneath the erosion level
(Thompson 1981) are proposed for the generation of some
of the periclines (e.g. North Hill Pericline, see Fig. 4).
Lateral thrust tips may be seen elsewhere in the Mendips
indicating that not all thrusts are laterally persistent
structures. On the north side of the North Hill Pericline
(543526) the Miners Arms Thrust (Green
emplaces Old Red Sandstone Carboniferous
to on
& Welch 1965)
Limestone, omitting Lower Limestone Shales (d’”). To the
west of the Stock Hill Fault (see the Bristol or Wells sheets)
this thrust is not exposed and the Lower Limestone Shales
are once more present, indicating a frontal tip (see Fig. 4).
A similar structure exists (490525) in a small fault that splays
from the South Western Overthrust (TJ. In a south-easterly
direction along this thrust the stratigraphic separation
decreases until at the tip there is no stratigraphic separation;
the thrust terminates and a tip line fold is present (495523).
A third example of a lateral thrust tip occurs on thrust T,,
the Tickenham Fault (485728). Stratigraphic separation
decreases in a north-easterly direction until it reaches zero
at a tip line fold.
Thrust faults across the whole Bristol-Mendip area can
be observed in map view to cross-cut lithostratigraphy in
both footwall and hangingwall along the exposed thrust
lengths, indicating the presence of lateral or oblique ramps
rather than simple frontal ramps or
reality, irregular thrust surfaces cross-cut sub-planar
flats (Butler 1982). In
lithostratigraphic interfaces to give a complex pattern of
hangingwall and footwall cut-off lines. Using hangingwall
stratigraphic cut off points along thrust T,, the Dinantian
stratigraphy preserved in the T, klippen, and the balanced
cross-sections (Figs 6 & 7), we have constructed a
hangingwall cut-off line map (Fig. 5). In plan, cut-off lines to
the north of thrust T, are now eroded and those to the south
of this thrust are buried. Assuming initially sub-planar
stratigraphic interfaces, the observed irregularities in the
cut-off lines reflect the presence of a large-scale oblique
ramp which becomes gradually frontal in the west of the
area. On this structure is superimposed a smaller scale
culmination above the North Hill Pericline, defined by
frontal and oblique ramps. The hangingwall cut-off map
(Fig. 5) has been used to estimate the angle of ramp climb
through the Dinantian stratigraphy.

B R I S T O L - M E N D I P F O R E L A N D T H R U S T B E L T 67
ORS
---_
NORTH 1
NlVER EAKER HILL PEN HILL
_ c
5 km
Fig. 4. Evolutionary diagram to explain the piggy-back nature of the North Hill Pericline
(NHP) and the East Harptree klippe (EHK). V = H scale.
Fig. 5. Stratigraphic cut-off map for the hangingwall of thrust T2. Buried cut-offs occur to the south and eroded
cut-offs to the north of the main outcrop of thrust T2. Dark stipple, Old Red Sandstone; clear, Carboniferous
Limestone (including Millstone Grit); light stipple, Coal Measures. BDP, Black Down Pericline; NHP, North Hill
Pericline, PHP, Pen Hill Pericline.
I

68 G . D . WILLIAMS & T. J. C H A P M A N
Strike-normal cross-sections
1.1 km. A comparison of the two sections A and B (Figs 6 &
Two approximately north-south balanced cross-sections, A
and B, have been constructed (section lines on Fig. 2) with
the relevant displacement-distance diagrams (Chapman &
Williams 1984) (Figs 6 & 7). The hangingwall cut-off map
(Fig. 5) indicates that thrust T, cuts up section northwards
through the stratigraphy at a maximum angle of 22" in the
east of the area and a minimum of 6" in the west. The angle
measurtd is that between stratigraphic interfaces in the
hangingwall and thrust surface T, on the strike-normal
sections. This implies a thin skinned geometry for the thrust
system. The amplitude of major asymmetric folds and the
sheet dip or fold envelope dip can be estimated from surface
dip data. The sheet dip defines the angle of dip of the basal
thrust fault, and the amplitude of major folds dictates the
thickness of sub Old Red Sandstone rocks involved in the
linked-thrust system on area balance arguments. Using these
criteria, the minimum depth to the basal thrust is estimated
to vary from 3 to 5 km and is displayed on Figs 6 and 7.
The western section, A (Fig. 6) shows that thrust T, cuts
7) shows a thicker basement in the thrust sheets in the west.
In the east, the Coal Measures of the Bristol and Somerset
Coalfields are exposed above a dkcollement involving only a
thin basement wedge. The depth to dCcollement across the
south of the area is consistent at 4.9 km and in the north it is
at a depth of 3.1 km representing a basal thrust presently
dipping south at 3". Section B (Fig. 7) has undergone a total
shortening of 27 km (38%) using the hangingwall cut-off of
base ORS by T, as a datum.
In both footwall and hangingwall, thrusts T, and T, are
at a higher stratigraphic level in the east (Fig. 6) than the
west (Fig. 7). This observation is supported by the T2
hangingwall cut-off diagram (Fig. 5) which illustrates
oblique ramp climb in a north-easterly direction. In reality
both T, and T3 are oblique hangingwall and footwall ramps,
climbing stratigraphy towards the NE and it is possible that
the en-echelon nature of the Mendip periclines may be, in
part, related to these thrusts.
up stratigraphy in the transport direction at a very shallow
angle. It is folded by the Blackdown Pericline and an
Strike-parallel cross-sections
associated syncline at Blackdown has subsequently been
eroded. This syncline plunges westwards and preserves d3
Two strike-parallel cross sections C & D (Fig. 8) have been
constructed, as viewed southwards against the thrust
limestone in the Churchill klippe to the west of section line movement direction. Major thrusts T1 to T4 are labelled,
A. The shallow angle of T, ramp climb means that thrust and it may be noted that older rocks are consistently
sheet T, once extended significantly farther north than displaced above younger rocks.
suggested by the present outcrop map. Thrust T3 is Section C (Fig. 8) illustrates a greater thickness of Lower
responsible for the generation of the Blackdown Pericline Palaeozoic basement in the west and the corresponding
and it possibly joins T, which forms a roof thrust. The Avon preservation of higher Carboniferous stratigraphy in the
Thrust, T4, and those to the north represent only minor east. Thrusts T, and T, exhibit numerous footwall and
displacements. The structural complexity at Portishead is hangingwall stratigraphic cut-offs and this corroborates their
due to pre-Pennant Measures (d6) folding and erosion. The interpretation as lateral or oblique ramps. A sequential
unconformable relationship of d6 to folded lower units may diagram (Fig. 9) illustrates how different hangingwall and
be seen on the restored section. Using the amplitude of footwall stratigraphies were juxtaposed along thrust T,. In a
major folds such as the Blackdown Pericline, it appears that layer-cake stratigraphy, the thrust trajectory will be
the thickness of pre-ORS basement above the basal thrust irregular if the thrust cuts up and down stratigraphy laterally
varies from 1.2 km in the north to 2.2 km in the south. The (Fig. sa). The irregularly truncated hangingwall stratigraphy
hangingwall cut-off of base ORS by thrust T2 may be used as will be transported in a foreland direction and may be
a datum point to calculate a total shortening of 19 km (38%) emplaced on an irregular footwall (Fig. 9b). Internal
for this section. Displacement-distance diagrams (Chapman hangingwall strain will take the form of culminations and
& Williams 1984) shown in Figs 6 & 7 record shortening depressions. Because a thrust will cut up stratigraphy in a
only to the north, and in the footwall, of thrust T,. This foreland direction, older rocks will be placed on younger
accounts for only approximately 20% shortening in both rocks (Fig. 9c). Further folding of both hangingwall and
cross-sections A and B (Figs 6 & 7), and this serves to footwall, plus the intervening thrust fault T2 has been caused
illustrate the magnitude of displacement on thrust T,. by movement on a lower thrust T, and a splay from T3, the
Backthrusts south of Nailsea propagate from thrust T4 and Miners Arms Thrust (Fig. 9d). Thrust T3 cuts down section
are responsible for small displacements which back rotate laterally, incorporating lower stratigraphy into the thrust
and steepen thrust T,. Cross sections on the 1 : 63,360 Bristol sheet in a westerly direction. This has generated a major
sheet show these faults as steep S-dipping normal faults. The N-S trending culmination which serves to fold the overlying
outcrop pattern of the Brockley Fault (470680) indicates a thrust T,. This culmination is the North Hill Pericline. The
northerly dip and supports its interpretation as a backthrust. general oblique NE climb of T, in its hangingwall is
The eastern section, B (Fig. 7) shows that thrust T2 cuts responsible for the en-echelon pattern of the Black Down
up section rapidly in the movement direction. It is folded and North Hill periclines.
above the Beacon Hill Pericline, and the small syncline to Section D (Fig. 8) shows thick a wedge of Lower
the north of Beacon Hill when projected eastwards is Palaeozoic rocks above the basal thrust in the west, and the
responsible for the preservation of the Luckington, Vobster basal detachment within ORS to the east. In the east of this
and Upper Vobster klippen. This section differs from A in section the N-S trending Coalpit Heath basin is seen to be
that there is only a thin wedge of pre-ORS basement above underlain by the proposed extension of the Malvern fault
the basal detachment and this wedge thins to zero beneath line. To the east of this, the ORS thins dramatically, and is
the Radstock Coalfield (points R and R' on deformed and
restored sections). To the north of point R, the basal thrust
underlain by Lower Palaeozoic basement. As with Section
C, the existence of Lower Palaeozoic rocks above a planar
fault is within the Old Red Sandstone sequence. The
basal thrust controls the present erosion level of thrust
maximum thickness of basement in the south of section B is culminations and depressions.

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70
G . D . WILLIAMS & T . J. C H A P M A N
. I F-----
I’ I

EAST
BRISTOL-MENDIP FORELAND THRUST BELT 71
5 ~~ km
l 1
C SECT. B SECT.A Cl
I
-
SECT. C
Fig. 8. Strike-parallel cross sections (C & D) across the Bristol-Mendip area. See Fig. 2 for the location of sections
Vertical = horizontal scale.
EAST WEST
-
Hangingwall
- -- -_
Discussion
T 2
We propose that the Mendip-Bristol area represents a thin
-33
d2
ORS'' d l 7!.!.!.1.!.! ..:. ................... I ! .--
.... : ...... . :, . ~ .............................................
. . .........,.....,............
. . . . . , . . . . m
. ~ . . . . .
skinned foreland thrust belt of Variscan age. The dominant
direction of thrust movement was south to north. Our
interpretation is based on surface lithostratigraphic and
T--_-- 2
structural observations, plus mine and borehole information
-_ Footwall . (see Bristol sheet) from which the balanced cross sections
'. - -
(Figs 6 & 7) were constructed. Evidence for the thin skinned
/--
...............................
I I I I I I I I I I I I I ...-. I I I. . I . I . ) . . ~ . ~ . . I . I. . I . I . . ~ . I...... . I ~ I I I ~ ~ , ~ ~ I
. . ;*.: . . . . . S . . . . . . . . .... . . . ............... . . .,.: :. .......... ,a,: . . . .:.. ::.I: ',: .. :. ..... :: 1 ................ nature of the belt is provided by the existence of
. . . . . . .
far-travelled klippen (e.g. Upper Vobster, Churchill). A
combination of hangingwall cut-off points and the preserved
stratigraphy in certain klippen have enabled the construction
of a hangingwall cut-off map (Fig. 5 ) for thrust T2. By
combining stratigraphic information with the position of
cut-off lines, calculated angles of frontal ramp climb
between 6" and 22" support the thin skinned interpretation.
Further support for this suggestion is obtained from the
sheet dip of the Lower Carboniferous rocks. We have
assumed that the basal thrust is subparallel to this sheet dip
in strike-normal sections. In similar forland thrust belts such
as the Appalachians (Gwinn 1964; Rodgers 1970) and the
Rockies (Price 1981) both of which are well constrained by
seismic data, the relationship between sheet dip and basal
detachment is confirmed. Our interpretation of the Variscan
structure of the area differs markedly however from that
Fig. 9. Hangingwall sequence diagram for thrust T2 and the
generation of the culmination beneath it, as viewed southwards
parallel to thrust movement direction. (a) preserved hangingwall
stratigraphy, (b) footwall stratigraphy, (c) juxtaposition of two
stratigraphies, and (d) generation of the culmination and folding of
given by Kellaway & Hancock (1983) who proposed a
folded dkcollement in Silurian sandstone and ?hale beneath
the Somerset and Bristol coalfields. Also, rather than
piggy-back thrusting with folded upper thrusts, Kellaway &
Hancock (1983) proposed lag faults for the numerous
thrust K?.
klippep in the Mendips.
WEST

72 G . D . WILLIAMS & T. J . CHAPMAN
the Somerset and Bristol coalfields. Also, rather than
piggy-back thrusting with folded upper thrusts, Kellaway &
Hancock (1983) proposed lag faults for the numerous
klippen in the Mendips.
The incorporation of Lower Palaeozoic basement wedges
into thrust sheets satisfies area balancing constraints. The
thickness of Lower Palaeozoic basement in the thrust sheets
calculated from observations of major fold amplitudes in
strike-normal sections reduces from south to north in
strike-normal sections and from west to east in strikeparallel
sections. The eastward decrease in basement
thickness explains why Lower Carboniferous and ORS
strata are exposed in the west and Upper Carboniferous in
the east. The Mendip periclines are asymmetric and verge
northwards, and their periclinal nature is due to thrust
culminations involving basement at depth. Their overall
en-echelon pattern may be related to the oblique climb of
two major thrusts (T2 and T3) towards the NE, and climb of
the basal detachment eastwards. The majority of exposed
thrusts are oblique hangingwall and footwall ramps.
Deformation which took place earlier than the Variscan
thrusting, is observed in the north of the area where Upper
Carboniferous Pennant Measures unconformably overlie
previously folded Lower Carboniferous and ORS rocks,
notably along the Lower Severn Axis. Such structures may
be related to major lineaments such as the Malvern Fault
Zone (Kellaway & Hancock 1983) or lesser faults such as
the ‘disturbances’ observed on the north crop of the South
Wales Coalfield (Owen 1971, 1983). The north-south
trending Coalpit Heath basin and Tortworth inlier are
almost certainly related to major pre-Variscan basement
lineaments. In terms of Variscan evolution it is possible that
whilst deformation was beginning in southern Cornwall and
Devon, in late Devonian to early Carboniferous times
(Dearman 1971), basement lineaments crossing the Variscan
foreland were activated as sinistral strike-slip faults giving
localized belts of deformation, and localized uplift and
subsidence. Piggy-back thrusting migrated northwards
(Shackleton et al. 1982; Coward & Smallwood 1984) and by
latest Carboniferous times the locus of thrusting was in the
Mendip area. The later, thin skinned thrusts cut across and
detached the early basement faults incorporating them into
the thrust sheets. The strike of thrusts T, and T, in the north
of the area may well be controlled by earlier, deeper faults
and they have probably undergone both strike-slip and
thrust displacements. The Variscan ‘front’ is a diffuse zone
in which thrust-type deformation dies out. It is likely to be
represented by a blind thrust that exists significantly farther
north than the previously suggested position of the Variscan
front (Hancock et al. 1983; Chadwick et al. 1984; Shackleton
1984). The possibility exists that early basement-controlled
structures such as the Usk and Tortworth inliers, and the
Forest of Dean syncline are all underlain by the northward
extension of the basal thrust represented on
C and D (Figs 6, 7 & 8).
sections A , B,
We would like to thank Paul Hancock for valuable discussion and
our colleagues at Cardiff and Plymouth for the same. Thanks are
also due to G. A. Kellaway for comments on the detailed geology
of the Bristol-Mendip area and to Mark Cooper and Geoff Tanner
for critical reviews of the manuscript. This paper is dedicated to the
innumerable undergraduates who have ‘done’ the Bristol sheet.
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BRISTOL-MENDIP
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BELT 73
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