- Email: [email protected]
An upper strain detachment model for the Ballard Fault: reply
CORRESPONDENCE
1987. Structural inversion : examples from the Alpine foreland and French Alps. Geotectonica Acta , 1,5-34. GRANT , N. T . 1990. Episodic discrete and distributed deformation: consequences and controls in a thrust culmination from the central Pyrenees. J. Struct. Geol., 12,835-50. HANCOCK, P. L. 1985. Brittle microtectonics: principles and practice. J . Struct. Geol., 7,437-57. HOUSE, M. R . 1989. Geology of the Dorset Coast. Geol. Ass. Field Guide , London. JONES, P. B. 1982. Oil and gas beneath east-dipping underthrust faults in the Alberta foothills. In (Powers , R. B.; ed .) Geologic Studies of the Cordilleran Thrust Belt. Rocky Mountain Assoc. Geol., 61-74. LAKE, S. D. 1985. The structure and evolution of the Wessex Basin in southern England. Unpubl Ph.D . thesis, University of Durham. LISTER, G. S. & P. F. WILLIAMS. 1979. Fabric development in shear zones: theoretic controls and
315
observed phenomena. J . Struct. Geo/. , 1,283-97. PHILIPS , W. J. 1964. The structures in the Jurassic and Cretaceous rocks on the Dorset coast between White Nathe and Mupe Bay. Proc. Geol. Ass., 75,373-405. RICH , J . L. 1934. Mechanics of low angle overthrust faulting illustrated by Cumberland thrust block, Virginia , Kentucky and Tennessee. Am. Ass . Petrol. Geol. Bull., 18, 1584-96. STONELEY, R . 1982. The structural development of the Wessex Basin. J. Geol. Soc. Lond. , 139, 543-54. - - & R. C. SELLEY. 1987. Petroleum habitat in south Dorset. In (Brook s, J . & K. Glennie ; eds). Petroleum Geology of North West Europe, Graham & Trotman, 139-48. SUPPE , J. 1983. Geometry and Kinematics of Fault-bend folding. Am. J. Sci. , 283,684-721. WILTSCHKO , D . & D. EASTMAN. 1983. Role of basement warps and faults in localising thrust fault ramps. Geol. Soc. Am. Mem. , 158,177-90.
An upper strain detachment model for the Ballard Fault: reply M. S. Ameen* and J. W. Cosgrove Department of Geology, Imperial College, Prince Consort Road, London, SW72BP
The Ballard Fault outcrops in the cliff section at Studland Bay where it cuts the steep limb of the Purbeck Isle-of-Wight monocline (Fig. 1). Ameen & Cosgrove (1990a) have interpreted the fault at this locality as part of a ramp associated with a northward propagating thrust. This interpretation has been questioned by Carter (1991) who argues that the fault forms part of a southward moving thrust. In order to resolve this disagreement it is useful to consider first the various models proposed for the formation of the Ballard Fault. The idea proposed by Carter (1991) that the Ballard Fault is a southward moving thrust or reverse fault is not new. It was previously proposed by Clarke (1837) whose cartoon of the generation of the Ballard fault is compared with that proposed by Carter in Fig. 2a & b. Strahan (1895) also considered the Ballard Fault to be a north dipping thrust associated with the formation of the monocline . Shotten & Taitt suggested independently to Arkell (1947) that the fault may be a local phenomenon occurring within the hinge region of the monocline. Their suggestion is shown schematically in Fig. 2c and has been termed the 'onion-scale adjustment faulting hypothesis' . The fault has also been interpreted as a normal fault (a gravity collapse structure, downthrow to the north, • Current Address: Geoscience Ltd, Silwood Park, Buckhurst Road, Ascot , Berkshire SLS 7QW
Fig. 2d) by Arkell (1936, 1947). The problems with this interpretation are briefly discussed by Ameen & Cosgrove (1990a) who argue that the intact state of the hanging wall does not support this idea. Seismic sections (e.g. Stoneley, 1982; Simpson, Gravestock , Ham, Leach & Thompson , 1987) show clearly that the Purbeck Isle-of-Wight monocline is a forced fold related to reverse dip-slip movement on a pre-existing normal fault in the basement. Ameen (1990) put forward the suggestion that the Ballard Fault represents an accommodation fault formed in association with the growth of the forced fold. This idea sprang from an experimental and field study of forced folding (Ameen, 1988, 1990, 1991) during the course of which it became apparent that a variety of meso- and macro-fractures can develop in association with the folding of cover rocks overlying a dip-slip basement fault (Fig. 2e). The macro-fractures (faults) are discussed by Ameen (1990) who considered the possibility that the Ballard fault represented one of the accommodation faults associated with the amplification of the Purbeck Isle-of-Wight monocline (Fig. 2e, fault 1). However, meso-fractures (faults) associated with the Ballard Fault and which are restricted to the footwall (Fig. 1), indicate a sense of movement opposite to that exhibited by fault 1 in Fig. 2e. This prompted Ameen & Cosgrove (1990a) to propose an alternative explanation for the formation of the Ballard Fault (Fig. 2f). Having briefly discussed the various models
316
C O R R E S P O N D E NCE
First Order Faul1s
s
N
Second Order Faults Systematic Meso Fractures Bedding
o, Horizontal Scale' Vertical Scale
Fig. 1. Section of part of the Ballard Cliff showing the Ballard North Fault (F) , the Ballard South Fault (F) and the meso-fractures (from Ameen & Cosgrove, 199Oa) .
proposed for the formation of the Ballard Fault the specific criticisms levelled by Carter (1991) against the model shown in Fig. 2f will be considered. From a detailed study of the meso-fractures in the Upper Chalk both in the vicinity of and away from the Ballard Fault (Ameen , 1988, 1990, 1991; Ameen & Cosgrove , 1990a & b) it is clear that some are developed uniformly throughout the steep limb of the monocline and others localised in a zone approximately 110 m wide beneath the Fault (Fig. 1). In addition the intensity of the locally developed meso-fractures decreases dramatically away from the fault. Ameen and Cosgrove (1990a) interpret this as indicating that they were intimately associated with the Ballard Fault and argue that any kinematic analysis of this fault should account for the formation of the meso-fractures. They argue that the orientation and sense of movement on the meso-fractures (which they suggest are R, Riedel shears) are compatible with a 'top to the north' movement on the Ballard fault. However , they recognise that some late , 'top to the south' movement has occurred along the Ballard fault as indicated by the deflection of the bedding and meso-fractures in the footwall . The latter movement they suggest is probably related to the amplification of the monocline during which time the part of the Ballard Fault at present exposed in the cliff section acted in the same sense as fault 1 in Fig. 2e. Carter disagrees with this interpretation and argues that the locally developed meso-fractures beneath the Ballard Fault represent small thrusts predating the Ballard Fault and which either predate the monocline or which formed during the initial stages of its amplification.
Small-scale thrusts of the type mentioned by Carter do exist and have been recorded , classified and discussed by Ameen (1988, 1991) and Ameen & Cosgrove (1990a section 3b, pp. 124-125 and Fig. 2d). However they differ from the meso-fractures beneath the Ballard Fault in that they are uniformly developed throughout the steep limb, i.e. are not restricted to the vicinity of the macro-fault. In addition they are smaller, show less displacement and are less intensely developed . Carter also criticizes the (northward thrusting) model of the Ballard Fault proposed by Ameen & Cosgrove (Fig. 2f) on geometric grounds arguing that the hanging wall cut off should be present to the north of the Ballard fault. It is acknowledged that this criticism would be valid if the geometry of the Purbeck Isle-of-Wight monocline was as simple as shown in the schematic diagram presented by Ameen & Cosgrove (1990a, Fig. 8 and this article Fig. 2f) to illustrate their model. However, as pointed out by Ameen (1990) and as implied by the profile of the monocline presented by Carter (Fig. 2b) , forced folds are frequently developed by rigid body rotation about several fixed hinges. It could therefore be argued that the absence of the zone of anomalous bedding dips reflects the fact that the sub-horizontal beds which outcrop north of the Ballard Fault are part of the ramp structure and situated in a rotated block that separates the vertical and horizontal limb of the monocline (Fig. 3a). It is interesting to note that in the geological section presented by Strahan (1895) (Fig. 3b) , a steepening of the Chalk occurs near its contact with the Tertiary rocks of Studland Bay. This may represent the point where the beds leave the
317
CORRESPONDENCE
s
s
N
N
I
~'
~i -------0-----I
(i) .
-------'--
--T-----: (ii)
----.,,-I
::;..,'
~~ "'+'
I
(iii)
N
S
N
(a)
(b)
s~ (e)
\~
~
~ nS~ {;;;)l~_
(e)
jf~
N
CRUSHED CHALK
Fig. 2. Various models proposed for the development of the Ballard Fault. (a) Clarke (1837) (b) Carter (1991) (e) Shotton & Taitt (in Arkell, 1947) (d) Arkell (1936, 1947) (e) Ameen (1990) and (f) Ameen & Cosgrove (1990a).
ramp and return to parallelism with the majority of the bedding. It is clear from the photograph amd geological section shown in Fig. 4a & b that there is an exact parallelism between the Ballard North Fault and the bedding in the hanging wall. Indeed this parallelism is the principal constraint imposed on the various models that have been proposed to account for the formation of the fault, (Fig. 2a, c, d, e & f). It is therefore surprising to observe that in his discussion of the bedding/fault relationship, Carter notes only a
sub-parallelism between the bedding on the hanging wall and the fault. This lack of absolute parallelism is compatible with the model he presents in Fig. 2b & 4c which requires that the fault cuts across bedding at Ballard cliff as it 'climbs' from one bedding plane at A to another at B (Fig. 4c & d), but incompatible with field observations by all previous workers (Fig. 4a & b). A second criticism that can be levelled against Carter's interpretation concerns the Ballard South Fault (Fl in Fig. 1). This, like the Ballard North
318
CORRESPONDENCE
s
N
/
BLOCK 4
Fixed Hinge Planes of Monocline (a)
s
I
.......................... " " -,
Ballard Down
N
-'. -."- , -, .... '
":~ ~ ~,~,,,.;i\~~).,\~;~l
'.
'.
':
\
\
\
-- - -- -- -- -- - - - - - - - --
(b)
Fig. 3. (a) A schematic representation of the folding of the Ballard Fault within the Chalk by the Purbeck-Isle of Wight monocline. The trace of the fault, which is assumed to be a southerly dipping thrust, is shown as a thick line. It is not known how much of the hanging wall shown on this diagram was removed during the erosion which preceded the deposition of the overlying Tertiary deposits. These younger rocks are not shown. Diagram not to scale. (b) A profile section of the Purbeck-Isle of White monocline at Ballard Cliff (from Strahan, 1895). The steepening of the dip of the chalk at the northern end of the section may correspond to the change in dip occurring from the ramp to flat shown in Fig. 3a.
Fault, he considers to be a southward moving, northerly dipping thrust (Fig. 2b & 4c). In order to support this suggestion he points to the occurrence of "Small-scale en echelon faults that have a shallower (gentler) dip angle than the main fault." He argues that "The orientation of these small scale faults suggests that they may represent synthetic Riedel (R) shears indicating a top to the south component of movement on the adjacent fault." Later in his paper, Carter points out that the Ballard North Fault also has locally developed sets of en echelon fractures in its footwall (Fig. 1). Ameen & Cosgrove (1990a) have interpreted these as second order fractures (Riedel
shears), related to the main fault and have used them to determine the sense of movement on this fault. Surprisingly, Carter is critical of this use, implying that these locally developed fractures pre-date the main fault and ought not to be used in this way. If they are unrelated to movement on the Ballard North Fault then their localised development in the footwall in a zone adjacent to the main fault requires explanation. In addition it seems unreasonable to use fractures developed locally around the Ballard South Fault to determine the sense of movement on this fault but to object to the use of such fractures in the analysis of the Ballard North Fault.
319
CORRESPONDENCE
N
s
N
s
,I .... "'I ..... )0......_
(c)
Fig. 4. (a) Photograph of the Ballard Fault (F) at Ballard cliff showing the parallelism between the fault and the bedding (B) in the hanging wall. (f) indicates second order faults. (b) The outline and dip of the bedding above and below the Ballard Fault at Ballard Cliff (Fig. 1 in Arkell, 1947). (c) Idealised section showing the relationship between the fault geometry proposed by Carter (1991, Fig. 6) and the present day coastal cliff section. The area enclosed by the square is shown in d. (d) The implications of the model for the Ballard Fault shown in (c) regarding the relationship between the bedding and the Ballard Fault. This should be compared with the observed relationship shown in (a) & (b).
As can be seen by comparing Fig. 2b and 2f, the critical difference between the two models is that Ameen & Cosgrove's model requires the beds above the Ballard fault to be older than those below it and Carter's model requires them to be younger. Studies of the palaeontology and stratigraphy of the Ballard
cliff coast section at present underway should resolve this problem. In addition, shallow seismic sections across the strike of the fault might reveal details of the fault/bedding relationships and enable the validity of the various models to be determined.
References AMEEN, M. S. 1988. Folding of layered cover due to dip-slip basement faulting. UnpubJ. Ph.D. thesis, Imperial College, University of London. - - 1990. Macrofaulting in the Purbeck-Isle of Wight monocline. Proc. Geol. Ass., 101,31-46. - - 1991. Strain pattern in the Purbeck-Isle of Wight monocline: a case study of folding due to dip-slip faulting in the basement. In (Bartholomew, M.J. et al., eds.) Characterisation and comparison of ancient (PrecambrianMesozoic) continental margins. Proceedings of the 8th International conference on basement tectonics, Butte, Montana, U.S.A. Reidel Publishing Co., Dordrecht, Holland (in press).
AMEEN, M. S. & J. W. COSGROVE, 199Oa. A kinematic analysis of the Ballard Fault, Swanage, Dorset. Proc. Geol. Ass., 101, 119-29. - - & - - 1990b. A kinematic analysis of mesofractures from Studland Bay, Dorset. Proc. Geol. Ass., 101, 303-14. ARKELL, W. J. 1936a. The tectonics of the Purbeck and Ridgeway faults in Dorset. Geol. Mag., 73, 56-73. - - 1936b. The tectonics of the Purbeck and ridgeway faults in Dorset. Geol. Mag., 73, 97-118. - - 1947. The geology of the country around Weymouth, Swanage, Corfe and Lulworth. Mem. Geol. Surv. Gt. BT. CARTER, D. C. 1991. An upper strain detachment model
320
CORRESPONDENCE
for the Ballard Fault: discussion. Proc. Geol. Ass., 102, 309-320. CLARKE, W. B. 1837. Illustrations of the geology of the south-east of Dorsetshire. Mag. Nat. Hist., n.s. 1,414-21, 461-69. ROWE, A. W. 1903. The Zones of the White Chalk of the English coast. II Dorset. Proc. Geol. Ass., 17, 1-76. SIMPSON, I. R, P. M. GRAVESTOCK, D. HAM, H.
LEACH & S. D. THOMPSON. 1987. Inversion tectonics of the Wessex Basin. In (Abstracts of the T.S.G. conference on Inversion Tectonics) March 1987, Roy. Soc. Lond. STONELEY, R 1982. The structural development of the Wessex Basin. J. Geol. Soc. Lond., 139,543-54. STRAHAN, A. 1895. On overthrusting of the Tertiary date in Dorset. Q. Jl. Geol. Soc. Lond., 51,549-62.
1987. Structural inversion : examples from the Alpine foreland and French Alps. Geotectonica Acta , 1,5-34. GRANT , N. T . 1990. Episodic discrete and distributed deformation: consequences and controls in a thrust culmination from the central Pyrenees. J. Struct. Geol., 12,835-50. HANCOCK, P. L. 1985. Brittle microtectonics: principles and practice. J . Struct. Geol., 7,437-57. HOUSE, M. R . 1989. Geology of the Dorset Coast. Geol. Ass. Field Guide , London. JONES, P. B. 1982. Oil and gas beneath east-dipping underthrust faults in the Alberta foothills. In (Powers , R. B.; ed .) Geologic Studies of the Cordilleran Thrust Belt. Rocky Mountain Assoc. Geol., 61-74. LAKE, S. D. 1985. The structure and evolution of the Wessex Basin in southern England. Unpubl Ph.D . thesis, University of Durham. LISTER, G. S. & P. F. WILLIAMS. 1979. Fabric development in shear zones: theoretic controls and
315
observed phenomena. J . Struct. Geo/. , 1,283-97. PHILIPS , W. J. 1964. The structures in the Jurassic and Cretaceous rocks on the Dorset coast between White Nathe and Mupe Bay. Proc. Geol. Ass., 75,373-405. RICH , J . L. 1934. Mechanics of low angle overthrust faulting illustrated by Cumberland thrust block, Virginia , Kentucky and Tennessee. Am. Ass . Petrol. Geol. Bull., 18, 1584-96. STONELEY, R . 1982. The structural development of the Wessex Basin. J. Geol. Soc. Lond. , 139, 543-54. - - & R. C. SELLEY. 1987. Petroleum habitat in south Dorset. In (Brook s, J . & K. Glennie ; eds). Petroleum Geology of North West Europe, Graham & Trotman, 139-48. SUPPE , J. 1983. Geometry and Kinematics of Fault-bend folding. Am. J. Sci. , 283,684-721. WILTSCHKO , D . & D. EASTMAN. 1983. Role of basement warps and faults in localising thrust fault ramps. Geol. Soc. Am. Mem. , 158,177-90.
An upper strain detachment model for the Ballard Fault: reply M. S. Ameen* and J. W. Cosgrove Department of Geology, Imperial College, Prince Consort Road, London, SW72BP
The Ballard Fault outcrops in the cliff section at Studland Bay where it cuts the steep limb of the Purbeck Isle-of-Wight monocline (Fig. 1). Ameen & Cosgrove (1990a) have interpreted the fault at this locality as part of a ramp associated with a northward propagating thrust. This interpretation has been questioned by Carter (1991) who argues that the fault forms part of a southward moving thrust. In order to resolve this disagreement it is useful to consider first the various models proposed for the formation of the Ballard Fault. The idea proposed by Carter (1991) that the Ballard Fault is a southward moving thrust or reverse fault is not new. It was previously proposed by Clarke (1837) whose cartoon of the generation of the Ballard fault is compared with that proposed by Carter in Fig. 2a & b. Strahan (1895) also considered the Ballard Fault to be a north dipping thrust associated with the formation of the monocline . Shotten & Taitt suggested independently to Arkell (1947) that the fault may be a local phenomenon occurring within the hinge region of the monocline. Their suggestion is shown schematically in Fig. 2c and has been termed the 'onion-scale adjustment faulting hypothesis' . The fault has also been interpreted as a normal fault (a gravity collapse structure, downthrow to the north, • Current Address: Geoscience Ltd, Silwood Park, Buckhurst Road, Ascot , Berkshire SLS 7QW
Fig. 2d) by Arkell (1936, 1947). The problems with this interpretation are briefly discussed by Ameen & Cosgrove (1990a) who argue that the intact state of the hanging wall does not support this idea. Seismic sections (e.g. Stoneley, 1982; Simpson, Gravestock , Ham, Leach & Thompson , 1987) show clearly that the Purbeck Isle-of-Wight monocline is a forced fold related to reverse dip-slip movement on a pre-existing normal fault in the basement. Ameen (1990) put forward the suggestion that the Ballard Fault represents an accommodation fault formed in association with the growth of the forced fold. This idea sprang from an experimental and field study of forced folding (Ameen, 1988, 1990, 1991) during the course of which it became apparent that a variety of meso- and macro-fractures can develop in association with the folding of cover rocks overlying a dip-slip basement fault (Fig. 2e). The macro-fractures (faults) are discussed by Ameen (1990) who considered the possibility that the Ballard fault represented one of the accommodation faults associated with the amplification of the Purbeck Isle-of-Wight monocline (Fig. 2e, fault 1). However, meso-fractures (faults) associated with the Ballard Fault and which are restricted to the footwall (Fig. 1), indicate a sense of movement opposite to that exhibited by fault 1 in Fig. 2e. This prompted Ameen & Cosgrove (1990a) to propose an alternative explanation for the formation of the Ballard Fault (Fig. 2f). Having briefly discussed the various models
316
C O R R E S P O N D E NCE
First Order Faul1s
s
N
Second Order Faults Systematic Meso Fractures Bedding
o, Horizontal Scale' Vertical Scale
Fig. 1. Section of part of the Ballard Cliff showing the Ballard North Fault (F) , the Ballard South Fault (F) and the meso-fractures (from Ameen & Cosgrove, 199Oa) .
proposed for the formation of the Ballard Fault the specific criticisms levelled by Carter (1991) against the model shown in Fig. 2f will be considered. From a detailed study of the meso-fractures in the Upper Chalk both in the vicinity of and away from the Ballard Fault (Ameen , 1988, 1990, 1991; Ameen & Cosgrove , 1990a & b) it is clear that some are developed uniformly throughout the steep limb of the monocline and others localised in a zone approximately 110 m wide beneath the Fault (Fig. 1). In addition the intensity of the locally developed meso-fractures decreases dramatically away from the fault. Ameen and Cosgrove (1990a) interpret this as indicating that they were intimately associated with the Ballard Fault and argue that any kinematic analysis of this fault should account for the formation of the meso-fractures. They argue that the orientation and sense of movement on the meso-fractures (which they suggest are R, Riedel shears) are compatible with a 'top to the north' movement on the Ballard fault. However , they recognise that some late , 'top to the south' movement has occurred along the Ballard fault as indicated by the deflection of the bedding and meso-fractures in the footwall . The latter movement they suggest is probably related to the amplification of the monocline during which time the part of the Ballard Fault at present exposed in the cliff section acted in the same sense as fault 1 in Fig. 2e. Carter disagrees with this interpretation and argues that the locally developed meso-fractures beneath the Ballard Fault represent small thrusts predating the Ballard Fault and which either predate the monocline or which formed during the initial stages of its amplification.
Small-scale thrusts of the type mentioned by Carter do exist and have been recorded , classified and discussed by Ameen (1988, 1991) and Ameen & Cosgrove (1990a section 3b, pp. 124-125 and Fig. 2d). However they differ from the meso-fractures beneath the Ballard Fault in that they are uniformly developed throughout the steep limb, i.e. are not restricted to the vicinity of the macro-fault. In addition they are smaller, show less displacement and are less intensely developed . Carter also criticizes the (northward thrusting) model of the Ballard Fault proposed by Ameen & Cosgrove (Fig. 2f) on geometric grounds arguing that the hanging wall cut off should be present to the north of the Ballard fault. It is acknowledged that this criticism would be valid if the geometry of the Purbeck Isle-of-Wight monocline was as simple as shown in the schematic diagram presented by Ameen & Cosgrove (1990a, Fig. 8 and this article Fig. 2f) to illustrate their model. However, as pointed out by Ameen (1990) and as implied by the profile of the monocline presented by Carter (Fig. 2b) , forced folds are frequently developed by rigid body rotation about several fixed hinges. It could therefore be argued that the absence of the zone of anomalous bedding dips reflects the fact that the sub-horizontal beds which outcrop north of the Ballard Fault are part of the ramp structure and situated in a rotated block that separates the vertical and horizontal limb of the monocline (Fig. 3a). It is interesting to note that in the geological section presented by Strahan (1895) (Fig. 3b) , a steepening of the Chalk occurs near its contact with the Tertiary rocks of Studland Bay. This may represent the point where the beds leave the
317
CORRESPONDENCE
s
s
N
N
I
~'
~i -------0-----I
(i) .
-------'--
--T-----: (ii)
----.,,-I
::;..,'
~~ "'+'
I
(iii)
N
S
N
(a)
(b)
s~ (e)
\~
~
~ nS~ {;;;)l~_
(e)
jf~
N
CRUSHED CHALK
Fig. 2. Various models proposed for the development of the Ballard Fault. (a) Clarke (1837) (b) Carter (1991) (e) Shotton & Taitt (in Arkell, 1947) (d) Arkell (1936, 1947) (e) Ameen (1990) and (f) Ameen & Cosgrove (1990a).
ramp and return to parallelism with the majority of the bedding. It is clear from the photograph amd geological section shown in Fig. 4a & b that there is an exact parallelism between the Ballard North Fault and the bedding in the hanging wall. Indeed this parallelism is the principal constraint imposed on the various models that have been proposed to account for the formation of the fault, (Fig. 2a, c, d, e & f). It is therefore surprising to observe that in his discussion of the bedding/fault relationship, Carter notes only a
sub-parallelism between the bedding on the hanging wall and the fault. This lack of absolute parallelism is compatible with the model he presents in Fig. 2b & 4c which requires that the fault cuts across bedding at Ballard cliff as it 'climbs' from one bedding plane at A to another at B (Fig. 4c & d), but incompatible with field observations by all previous workers (Fig. 4a & b). A second criticism that can be levelled against Carter's interpretation concerns the Ballard South Fault (Fl in Fig. 1). This, like the Ballard North
318
CORRESPONDENCE
s
N
/
BLOCK 4
Fixed Hinge Planes of Monocline (a)
s
I
.......................... " " -,
Ballard Down
N
-'. -."- , -, .... '
":~ ~ ~,~,,,.;i\~~).,\~;~l
'.
'.
':
\
\
\
-- - -- -- -- -- - - - - - - - --
(b)
Fig. 3. (a) A schematic representation of the folding of the Ballard Fault within the Chalk by the Purbeck-Isle of Wight monocline. The trace of the fault, which is assumed to be a southerly dipping thrust, is shown as a thick line. It is not known how much of the hanging wall shown on this diagram was removed during the erosion which preceded the deposition of the overlying Tertiary deposits. These younger rocks are not shown. Diagram not to scale. (b) A profile section of the Purbeck-Isle of White monocline at Ballard Cliff (from Strahan, 1895). The steepening of the dip of the chalk at the northern end of the section may correspond to the change in dip occurring from the ramp to flat shown in Fig. 3a.
Fault, he considers to be a southward moving, northerly dipping thrust (Fig. 2b & 4c). In order to support this suggestion he points to the occurrence of "Small-scale en echelon faults that have a shallower (gentler) dip angle than the main fault." He argues that "The orientation of these small scale faults suggests that they may represent synthetic Riedel (R) shears indicating a top to the south component of movement on the adjacent fault." Later in his paper, Carter points out that the Ballard North Fault also has locally developed sets of en echelon fractures in its footwall (Fig. 1). Ameen & Cosgrove (1990a) have interpreted these as second order fractures (Riedel
shears), related to the main fault and have used them to determine the sense of movement on this fault. Surprisingly, Carter is critical of this use, implying that these locally developed fractures pre-date the main fault and ought not to be used in this way. If they are unrelated to movement on the Ballard North Fault then their localised development in the footwall in a zone adjacent to the main fault requires explanation. In addition it seems unreasonable to use fractures developed locally around the Ballard South Fault to determine the sense of movement on this fault but to object to the use of such fractures in the analysis of the Ballard North Fault.
319
CORRESPONDENCE
N
s
N
s
,I .... "'I ..... )0......_
(c)
Fig. 4. (a) Photograph of the Ballard Fault (F) at Ballard cliff showing the parallelism between the fault and the bedding (B) in the hanging wall. (f) indicates second order faults. (b) The outline and dip of the bedding above and below the Ballard Fault at Ballard Cliff (Fig. 1 in Arkell, 1947). (c) Idealised section showing the relationship between the fault geometry proposed by Carter (1991, Fig. 6) and the present day coastal cliff section. The area enclosed by the square is shown in d. (d) The implications of the model for the Ballard Fault shown in (c) regarding the relationship between the bedding and the Ballard Fault. This should be compared with the observed relationship shown in (a) & (b).
As can be seen by comparing Fig. 2b and 2f, the critical difference between the two models is that Ameen & Cosgrove's model requires the beds above the Ballard fault to be older than those below it and Carter's model requires them to be younger. Studies of the palaeontology and stratigraphy of the Ballard
cliff coast section at present underway should resolve this problem. In addition, shallow seismic sections across the strike of the fault might reveal details of the fault/bedding relationships and enable the validity of the various models to be determined.
References AMEEN, M. S. 1988. Folding of layered cover due to dip-slip basement faulting. UnpubJ. Ph.D. thesis, Imperial College, University of London. - - 1990. Macrofaulting in the Purbeck-Isle of Wight monocline. Proc. Geol. Ass., 101,31-46. - - 1991. Strain pattern in the Purbeck-Isle of Wight monocline: a case study of folding due to dip-slip faulting in the basement. In (Bartholomew, M.J. et al., eds.) Characterisation and comparison of ancient (PrecambrianMesozoic) continental margins. Proceedings of the 8th International conference on basement tectonics, Butte, Montana, U.S.A. Reidel Publishing Co., Dordrecht, Holland (in press).
AMEEN, M. S. & J. W. COSGROVE, 199Oa. A kinematic analysis of the Ballard Fault, Swanage, Dorset. Proc. Geol. Ass., 101, 119-29. - - & - - 1990b. A kinematic analysis of mesofractures from Studland Bay, Dorset. Proc. Geol. Ass., 101, 303-14. ARKELL, W. J. 1936a. The tectonics of the Purbeck and Ridgeway faults in Dorset. Geol. Mag., 73, 56-73. - - 1936b. The tectonics of the Purbeck and ridgeway faults in Dorset. Geol. Mag., 73, 97-118. - - 1947. The geology of the country around Weymouth, Swanage, Corfe and Lulworth. Mem. Geol. Surv. Gt. BT. CARTER, D. C. 1991. An upper strain detachment model
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for the Ballard Fault: discussion. Proc. Geol. Ass., 102, 309-320. CLARKE, W. B. 1837. Illustrations of the geology of the south-east of Dorsetshire. Mag. Nat. Hist., n.s. 1,414-21, 461-69. ROWE, A. W. 1903. The Zones of the White Chalk of the English coast. II Dorset. Proc. Geol. Ass., 17, 1-76. SIMPSON, I. R, P. M. GRAVESTOCK, D. HAM, H.
LEACH & S. D. THOMPSON. 1987. Inversion tectonics of the Wessex Basin. In (Abstracts of the T.S.G. conference on Inversion Tectonics) March 1987, Roy. Soc. Lond. STONELEY, R 1982. The structural development of the Wessex Basin. J. Geol. Soc. Lond., 139,543-54. STRAHAN, A. 1895. On overthrusting of the Tertiary date in Dorset. Q. Jl. Geol. Soc. Lond., 51,549-62.