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Deformed micaceous deposits in the Downtonian of the Llandeilo region, South Wales
Deformed Micaceous Deposits in the Downtonian of the Llandeilo Region, South Wales by JOHN F. POTIER Received January 1966; taken as read 5 November 1966
CONTENTS page 277
I. INTRODUCTION
2. 3. 4.
277 280 282 286 286
FIELD OCCURRENCE MICROSCOPIc DETAILS ANALYSIS OF STRUCTURES ACKNOWLEDGMENTS REFERENCES
ABSTRACT: Bedding plane crenulations developed in highly micaceous sediments of the Downtonian rocks of South Wales are described. A tectonic origin is postulated for the structures and a comparison made between their formation and that of normal drag folds.
1. INTRODUCTION 1907, T. C. Cantrill (in Strahan et al., 51) recorded that the walls of Cil-maen-llwyd quarry, a locality situated two and a half miles ESE. of the town of L1andeilo,Carmarthenshire (666207), 1 were 'formed of bedding
IN
planes, which, 'here and there show minute wrinkling parallel to the strike'. Since that time no published explanation has been offered as to the origin of these structures. Cil-maen-llwyd quarry, and other localities where comparable minute crenulations are exhibited, occur on the southern limb of the Towy anticline. In the vicinity of L1andeilo this limb, in contrast to the northern limb, is very steeply inclined (Fig. 1). 2. FIELD OCCURRENCE A locality which occurs approximately 750 ft. from the south-west extremity of Cil-maen-llwyd quarry (66292056), and about 30 ft. from the base of the Long Quarry Bed formation (Potter & Price, 1965), displays an unusual sequence of sedimentary lithologies in which a laminated mica band up to one and a half in. in thickness is developed. Wrinkled lamination parallel to the strike of the rocks is well exhibited on the bedding 1
All National Grid References lie within the 100 km. square 22 (SN.).
277
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Fig. I. Sketch-map and section of the rocks in the vicinity of Llandeilo, Carmarthenshire
DEFORMED MICACEOUS DEPOSITS
279
planes of this micaceous band (plate 9, A). The wrinkling affects the entire thickness of the band but appears to decrease in intensity towards the upper and lower contacts of the bed. It assumes a sub-parallel en echelon pattern, and the individual fold axes rarely exceed 3 in. in length. Within the Long Quarry Bed exposures to the west of Cil-maen-Ilwyd quarry, plicated and wedging micaceous bands of this nature are occasionally developed into deposits as much as 6 in. thick. Under examination, such rocks can be seen to be composed of compacted laminar minerals, these greatly predominating over the normal siliceous components of a sandstone. Frequently 90 per cent of the rock is composed of micaceous minerals-particularly muscovite. Generally the crenulated micaceous beds are sandwiched between hard and competent arenaceous deposits and their contact surfaces are nearly smooth. Unfortunately, no slickensides have been observed on the surfaces. Instances of microfolded micaceous strata are fairly common at other Long Quarry Bed localities and have even been observed in thin micaceous seams in the Downtonian, Capel Berach Beds. Some typical recordings, tabulated from east to west through the Llandeilo region, are displayed in Table I.
TABLE
I. Some Localities Exhibiting Plicated Micaceous Deposits
Locality and Horizon
I. Cil-maen-llwyd Quarry Long Quarry Beds (Locality cited in text) 2. Cil-maen-llwyd Quarry Long Quarry Beds 3. Quarry, E. end of Grip Wood Long Quarry Beds 4. Quarry, E. of L1andeiloAmmanford Road Long Quarry Beds 5. L1andeilo-Ammanford Road Long Quarry Beds 6. Quarry, 250 yds, E. of Llethr-garw Long Quarry Beds 7. Quarry, 200 yds. N. of Llygad-yr-haul Capel Berach Beds
Grid Reference
Strike
Dip (in a southerly direction)
Pitch of Microfold Lineation in Bedding Plane
66292056
69°
89°
9tO to W.
66262052
74°
Horizontal
64271980
85°
61481914
92°
Inverted by 8° Inverted by 9° Inverted by 8°
61441913
77°
88°
Horizontal
61051906
95°
Inverted by 6°
31° to W.
57781882
86°
87°
lJO to W.
1° to W. 12° to W.
280
JOHN F. POTTER
3. MICROSCOPIC DETAILS A microscopic examination of mica flakes separated from the highly micaceous Long Quarry Bed deposits clearly indicates that they are very well rounded and obviously detrital grains. Suggestions that the micaceous minerals might have been created by metamorphic recrystallisation processes or by authigenesis are not acceptable. According to Krynine (1940, 82), rounded mica flakes are a feature which indicate depositional conditions 'of sluggishly moving currents with gentle to-and-fro motion'. A separation of muscovite flakes viewed between crossed nicols (Plate 9, B) shows that the well-rounded flakes have moderately birefringent grain boundaries, evidently a result of bruising suffered by collisions experienced prior to final sedimentation. Such attrition suggests the existence of more turbulent current conditions than those envisaged by Krynine. Microscopic sections of the extremely friable, crenulated micaceous deposits, although difficult to prepare, are of particular interest. Sections cut normal to the lamination and microfold axes show both the rock and the individual mica flakes to have suffered very considerable contortion. Areas occur in which sub-parallel kink planes bound kink bands which are symmetrically orientated to them. These are more frequently towards the central portions of sectioned mica-rich beds (Fig. 2). The kink planes, although they sometimes diverge, are commonly orientated normal to the bedding planes (Plate 10, C). Paterson (1964, 30) has suggested that this type of structure represents an intense degree of deformation and severe shortening in a direction parallel to the rock lamination. In other domains of the rock, bundles of micaceous flakes have been deformed by bend gliding. The two deforming processes of kinking and bend gliding are frequently transitional into each other in a manner comparable to that originally displayed by Orowan (1942) in metal wire (plate 10,D). In some instances the micaceous minerals in the domain of bend gliding associated with a kink plane tend to form into conjugate folds (Plate 11, E). The intensity of deformation through the thickness of the micaceous beds is surprisingly variable. Large areas of the rock do not exhibit kink planes, but only display nearly geometrically symmetrical plications. In these areas, the micaceous minerals, acting as relatively strong, competent and yet elastic units, form microfolds which more generally approximate to a parallel or concentric pattern of folding. For this reason, deformation in the cuspate cores ofthe parallel microfolds is often complex (Plate 10,D). Conditions of intense deformation in the microfold cores in certain cases produce 'neck' structures (Hills, 1963, 244), where compression causes a break-out or core material by fracture (Plate 11, F). Microscopically there is little evidence for any shearing-out of the microfold limbs of the rock by processes which result in strain-slip cleavage (Turner & Weiss, 1963, 98). Signs of nco-crystallisation of the
PROC. GEOL. ASS., VOL. 78 (1967)
PLATE 9
A. Structural plications on a micaceous band, Cil-maenIIwyd quarry, L1andeilo (photograph by E. Martin, Esq.)
B. Separation of muscovite flakes showing birefringent grain boundaries (crossed nicols, x 38) [To face p. 280
PROC. GEOL. ASS., VOL. 78 (1967)
PLATE 10
C. Sub-parallel kink planes in the micaceous deposit (crossed nicols, x 65)
D. Transition of a kink plane into an area of bend gliding (ordinary light, x 54)
PROC. GEOL. ASS. , VOL . 78 ( 196 7)
PLATE 11
E. Dom ain of co nj uga te microfoldin g sit uat ed between regions o f norma l bend glidin g a nd kin kin g (or dina ry light , x 33)
F . Break -out o f core material in parall el microfold \0 form a 'n eck ' struc ture (o rdina ry light , x 45)
G. Det a il of several mu sco vite flak es, showing microscopic kin k plane passing into zone of bend gliding (o rd ina ry light , x 160)
DEFORMED MICACEOUS DEPOSITS
281
flaky minerals along incipient shear planes are similarly absent. Axial plane extension in the crenulations seems to have taken place mainly by the process of kinking. de Sitter (1956, 217), in reference to accordionfolding, has shown such extension develops when kink planes are present. The kink planes in the strongly micaceous and crenulated Long Quarry Bed sediments are also axial planes which bisect the kink folds. Dewey (1965, 461) recently considered this type of occurrence to be a special case of kinking only possible in reverse kink-zones. Rotation of the kink
~- E
< Fig. 2. Diagrammatic representation of narrow (e.g. half an inch) mica band showing possible occurrence of structures illustrated in Plates 9 to 11 (C to F respectively)
JOHN F. POTTER
282
a.
b.
c.
d.
Fig. 3. Microscopic distortion and fracture of mica flakes a. Bend gliding in a slightly deformed flake of muscovite showing the development of cleavage parting (approximate magnification x 48) b. Fracture and bend gliding of flakes (approximate magnification x 28) c. Sub-parallel kink planes and cleavage parting in a muscovite flake (approximate magnification x 26) d. Sub-parallel kink planes or fractures in a muscovite flake. The flake is splayed out at the crest of one kink plane (approximate magnification x 26)
bands occurs by slip along the layers and cleavage of the micaceous minerals. Individual mica flakes show comparable deformations to those seen in the rock as a whole (Fig. 3). Microscopic kink planes again pass into zones of bend gliding (Plate 11, G). Where the rock has been thrown into tightly folded corrugations, individual flakes may be fractured, or very rarely splayed out, at the points of greatest flexure. The degree of bending required to fracture a mica flake was extremely variable; presumably it was related to the confining forces and other factors that controlled the elasticity of each grain. However, flakes bent to form an angle of less than 75° are invariably fractured. Bend gliding in individual flakes or bundles of flakes, of the type exhibited in Fig. 3a, where the cleavage parting of the bent flakes is developed, naturally tends to thicken the crests and relatively attenuate the limbs of the microfolds. Crenulation in the rock does not appear to have been strongly influenced by the more equidimensional siliceous grains, even when these grains reach a comparatively large size; nor do these grains show any evidence of rotation independent of the surrounding rock matrix. Frequently pressuresolution has taken place between the grain boundaries of the siliceous and flaky minerals, both materials displaying signs of solution, but the former being more extensively affected (cf. Nicholson, 1966). 4. ANALYSIS OF STRUCTURES In a recent paper Williams (1961) illustrated that in instances of similar folding the more incompetent beds accommodate themselves into the
DEFORMED MICACEOUS DEPOSITS
283
interspaces by flow as a result of considerable changes in the shape of the fold cross-section during the development of the fold. Drag folds within the incompetent layers were accredited to the differential movement between the flow laminae in a direction sub-parallel to the axial plane of the principal fold structure. These drag folds are best observed in thin competent layers (Ramberg, 1963, 97) within the incompetent unit. They have an asymmetrical shape, and have long been recognised as important structural features. The associated regional structure has frequently been inferred from their attitude (as in Leith, 1923; Billings, 1958; and others). A
B.
Fig. 4. A. Folded competent (stippled) and incompetent beds (partly after Williams, 1961)
B. Folded competent beds, with inset of lenticular micaceous band. Arrows
indicate direction in which material tends to move
With parallel or concentric folding the beds undergo different confining forces and a different type of deformation (Fig. 4). Bedding plane slip must take place and flexural-slip folding of this type has often been cited (as Hills, 1963, 227). Normally, bedding plane slip would occur, as the name indicates, along the bedding planes of successive competent units rather than within a bed itself, for if an incompetent unit were involved similar folding would be produced. In the Long Quarry Beds the micaceous bands offer the rare example in which bedding plane slip has occurred within the unit. In this instance, there has been little variation in thickness of the confined accommodating unit during deformation. The mica flakes at the top and bottom of the micaceous bed are parallel to the bedding planes, and slip without evident contortion has taken place along their surfaces. More remote from the margins of the bed the mica flakes become wavy and at the centre of the micaceous layer crenulations are well developed. Geometrically these crenulations are almost symmetrical; they do not show the pronounced asymmetry that one would expect on the flanks of a layer fold structure (Wilson, 1961, 505, and fig. 37), and their axial planes are radially orientated with respect to the fold as a whole. It would appear that during the formation of the monoclinal Towy anticline, micaceous deposits interbedded with competent sandstones in
2i4
JOHN F. POTTER
the Long Quarry Beds acted as units in which bedding plane slip took place. The resulting movement created irrespective of orientation, symmetrical or nearly symmetrical, crenulations on the bedding plane laminations of the micaceous beds. The axial planes of the crenulations, seen in the steeply dipping southern limb of the anticline, are radial to the anticline fold axis. The microfold axes in all the plicated micaceous beds examined were approximately horizontal. This would imply that the major structural feature in the vicinity of Llandeilo, and in particular the southern limb of the Towy anticline, is unlikely to be affected by more than a few degrees of plunge. Any small angle of plunge that does affect the fold limb is towards the west. There appears to be no direct relationship between the size of the microfolds in the rock and the lateral dimensions of the individual micaceous minerals; nor does it seem likely that included siliceous grains in the deposit created irregularities in the laminar flow lines to produce the crenulations. It would seem possible that the first production of microfolds or plications might result from the tendency to develop a wave surface at the boundary between the flaky materials moving relatively in opposite directions (Lamb, 1932, 373 et seq.; Biot, 1963). The production of experimental folding in rocks (Paterson & Weiss, 1962; Paterson, 1964) clearly shows that comparable structures can be produced by compressing schistose rocks in a direction approximately parallel to their schistosity. In the Long Quarry Beds a deforming process such as this latter would have been assisted by the lenticular form of the micaceous bands. Biot (1964, 1965) has recently presented a theoretical study of the type of deformation seen in the Long Quarry Bed micaceous bands, where multilayers are internally buckled under rigid confinement. He determined that the dominant wavelength of the mica crenulation should be about .fed = 1.90 ~hI H
where, hI is the distance between the folding slip planes within the mica and H is the total thickness of the confined micaceous layer.
Both .fed 2 and H2 can be directly measured so that it is feasible to calculate a mean value for hi, For the example at Cil-maen-llwyd quarry cited in the present text, where H = 36 mm.,.fed = 4.3 mm.; from which it is possible to determine that hi is approximately 0.14 rom. The approximate mean value of hi for those field measurements illustrated in Fig. 5 (Table II) is 0.26 rom. These figures in excess of the average thickness of the mica 2 The observed values for these in the crenulated micaceous bands will be slightly dissimilar to those in Biot's equation which is intended to be more applicable to the earlier phases of deformation. The creation of kink folds would especially augment this difference.
285
DEFORMED MICACEOUS DEPOSITS
TABLE II. Observed mean values of ~d, andcalculated values of ~H and hr for the various Long Quarry Bed micaceous bands listed in Table I
Locality (see Table I) I 2 2 3 4 4 4 5 5 5 5 5 5 6 6 6
Mean ~d mm. 4.3 1.75 1.5 2.5 4.0 4.0 10.5 8.0 1.25 1.75 2.5 4.5 1.25 2.0 1.5 2.0
lii
Approx. H
mm.
( hl=
6.0 3.4 1.7 3.0 4.8 4.2 6.9 5.6 1.4 2.6 2.6 4.1 1.7 2.2 1.4 1.9
36.0 11.6 3.0 9.0 23.0 18.0 47.0 31.0 2.0 7.0 7.0 17.0 3.0 5.0 2.0 3.5
~d
)
1.90r:m~H
2
0.14 0.D7 0.21 0.19 0.19 0.25 0.64 0.56 0.22 0.13 0.26 0.33 0.15 0.23 0.32 0.31 Mean hI = 0.26
12
11 X 10
q X
Mean
Ld
b
mm.
x x
.i o
x
x
x x x x x
3
F~
Mean Fig. 5. Graph of observed mean values of ~d plotted against the mean values of ~ii for tile various Long Quarry Bed micaceous bands listed in Table 1
286
JOHN F. POTTER
flakes and suggests that movement has not taken place between all the flakes present. In the micaceous layers in the Long Quarry Beds hi is variable and difficult to determine. It may represent the distance between cleavage partings, or the thickness of individual flakes or stacks of flakes. Presumably the properties of the micas are sufficiently similar in all the crenulated bands to conclude that approximately
.?d ex: {if This conclusion appears to be confirmed by field observation and measurement (Fig. 5). ACKNOWLEDGMENTS The author wishes to thank Dr. M. A. Biot, Dr. D. J. Maull, Prof. J. G. Ramsay and Dr. E. Williams for their advice and discussion. Thanks are also due to Dr. Gilbert Wilson who was kind enough to read and criticise a draft of the manuscript.
REFERENCES BILLINGS, M. P. 1958. Structural Geology. New Jersey. BlOT, M. A. 1963. Internal buckling under initial stress in finite elasticity. Proc, R. Soc. A, 273, 306-28. - - - . 1964. Theory of internal buckling of a confined multilayered structure. Bull. geol. Soc. Am., 75, 563-8. - - - . 1965. Further development of the theory of internal buckling of multilayers. Bull. geol. Soc. Am., 76, 833-40. DEWEY, J. F. 1965. Nature and Origin of Kink-bands. Tectonophysics, 1,459-94. HILLS, E. S. 1963. Elements of Structural Geology. London. KRYNINE, P. D. 1940. Petrology and Genesis of the Third Bradford Sand. Pennsylvania State College Bulletin, 29, 82. LAMB, H. 1932. Hydrodynamics. London. LEITH, C. K. 1923. Structural Geology. New York. NICHOLSON, R. 1966. Metamorphic Differentiation in Crenulated Schists. Nature, Lond. 209, 68-9. PATERSON, M. S. 1964. Experimental Deformation of Rock. Discovery; 25, 27-31. - - - . & L. E. WEISS, 1962. Experimental folding in rocks. Nature, Lond. 195, 1046-8. POTTER, J. F. & J. H. PRICE, 1965. Comparative Sections through Rocks of Ludlovian -Downtonian Age in the Llandovery and Llandeilo Districts. Proc, Geol. Ass., 76, 372-404. OROWAN, E. 1942. A Type of Plastic Deformation New in Metals. Nature, Land. 149, 643-4. RAMBERG, H. 1963. Evolution of Drag Folds. Geol. Mag., 100, 97-106. de SITTER, L. U. 1956. Structural Geology, l st ed. London. STRAHAN, A., T. C. CANTRILL, E. E. L. DIXON & H. H. THOMAS, 1907. The Geology of the South Wales Coal-field. Part VII. The Country around Ammanford.
Mem, Geol. Surv, U.K. TURNER, F. J. & L. E. WEISS, 1963. Structural Analysis of Metamorphic Tectonites. New York.
DEFORMED MICACEOUS DEPOSITS
E. 1961. The Deformation of Confined, Incompetent Layers in Folding. Geol. Mag., 98, 317-23. G. 1961. The Tectonic Significance of Small Scale Structures, and their Importance to the Geologist in the Field. Annis. Soc. geol. Belg., 84, 423-548.
WILLIAMS, WILSON,
287
J. F. Potter Norwood Technical College Knight's Hill London S.E.27
CONTENTS page 277
I. INTRODUCTION
2. 3. 4.
277 280 282 286 286
FIELD OCCURRENCE MICROSCOPIc DETAILS ANALYSIS OF STRUCTURES ACKNOWLEDGMENTS REFERENCES
ABSTRACT: Bedding plane crenulations developed in highly micaceous sediments of the Downtonian rocks of South Wales are described. A tectonic origin is postulated for the structures and a comparison made between their formation and that of normal drag folds.
1. INTRODUCTION 1907, T. C. Cantrill (in Strahan et al., 51) recorded that the walls of Cil-maen-llwyd quarry, a locality situated two and a half miles ESE. of the town of L1andeilo,Carmarthenshire (666207), 1 were 'formed of bedding
IN
planes, which, 'here and there show minute wrinkling parallel to the strike'. Since that time no published explanation has been offered as to the origin of these structures. Cil-maen-llwyd quarry, and other localities where comparable minute crenulations are exhibited, occur on the southern limb of the Towy anticline. In the vicinity of L1andeilo this limb, in contrast to the northern limb, is very steeply inclined (Fig. 1). 2. FIELD OCCURRENCE A locality which occurs approximately 750 ft. from the south-west extremity of Cil-maen-llwyd quarry (66292056), and about 30 ft. from the base of the Long Quarry Bed formation (Potter & Price, 1965), displays an unusual sequence of sedimentary lithologies in which a laminated mica band up to one and a half in. in thickness is developed. Wrinkled lamination parallel to the strike of the rocks is well exhibited on the bedding 1
All National Grid References lie within the 100 km. square 22 (SN.).
277
N
.....
00
.....
o
::t Z "r'!
t
~
(( ".'
'"-i
o
i3
-i
MILES 0 12 3 4! 5I «!! I I KILO METRES
m ;:0
2,50
C.N1.L. -, CIL-MAEN- LLWvOOUARRY
~~ o T
.. -~
ORDOVICIAN
I \\ '::.::.,f,.0, ·°'0'·
--~-
~~\
'.0'.
Q: '0' ~
~
t:>...::>J
LLANDOVERY 'WENLoo<
r:"'1
~
LUDLOW
fa
DEVONIAN
[ill
CARBONIFER OUS
Fig. I. Sketch-map and section of the rocks in the vicinity of Llandeilo, Carmarthenshire
DEFORMED MICACEOUS DEPOSITS
279
planes of this micaceous band (plate 9, A). The wrinkling affects the entire thickness of the band but appears to decrease in intensity towards the upper and lower contacts of the bed. It assumes a sub-parallel en echelon pattern, and the individual fold axes rarely exceed 3 in. in length. Within the Long Quarry Bed exposures to the west of Cil-maen-Ilwyd quarry, plicated and wedging micaceous bands of this nature are occasionally developed into deposits as much as 6 in. thick. Under examination, such rocks can be seen to be composed of compacted laminar minerals, these greatly predominating over the normal siliceous components of a sandstone. Frequently 90 per cent of the rock is composed of micaceous minerals-particularly muscovite. Generally the crenulated micaceous beds are sandwiched between hard and competent arenaceous deposits and their contact surfaces are nearly smooth. Unfortunately, no slickensides have been observed on the surfaces. Instances of microfolded micaceous strata are fairly common at other Long Quarry Bed localities and have even been observed in thin micaceous seams in the Downtonian, Capel Berach Beds. Some typical recordings, tabulated from east to west through the Llandeilo region, are displayed in Table I.
TABLE
I. Some Localities Exhibiting Plicated Micaceous Deposits
Locality and Horizon
I. Cil-maen-llwyd Quarry Long Quarry Beds (Locality cited in text) 2. Cil-maen-llwyd Quarry Long Quarry Beds 3. Quarry, E. end of Grip Wood Long Quarry Beds 4. Quarry, E. of L1andeiloAmmanford Road Long Quarry Beds 5. L1andeilo-Ammanford Road Long Quarry Beds 6. Quarry, 250 yds, E. of Llethr-garw Long Quarry Beds 7. Quarry, 200 yds. N. of Llygad-yr-haul Capel Berach Beds
Grid Reference
Strike
Dip (in a southerly direction)
Pitch of Microfold Lineation in Bedding Plane
66292056
69°
89°
9tO to W.
66262052
74°
Horizontal
64271980
85°
61481914
92°
Inverted by 8° Inverted by 9° Inverted by 8°
61441913
77°
88°
Horizontal
61051906
95°
Inverted by 6°
31° to W.
57781882
86°
87°
lJO to W.
1° to W. 12° to W.
280
JOHN F. POTTER
3. MICROSCOPIC DETAILS A microscopic examination of mica flakes separated from the highly micaceous Long Quarry Bed deposits clearly indicates that they are very well rounded and obviously detrital grains. Suggestions that the micaceous minerals might have been created by metamorphic recrystallisation processes or by authigenesis are not acceptable. According to Krynine (1940, 82), rounded mica flakes are a feature which indicate depositional conditions 'of sluggishly moving currents with gentle to-and-fro motion'. A separation of muscovite flakes viewed between crossed nicols (Plate 9, B) shows that the well-rounded flakes have moderately birefringent grain boundaries, evidently a result of bruising suffered by collisions experienced prior to final sedimentation. Such attrition suggests the existence of more turbulent current conditions than those envisaged by Krynine. Microscopic sections of the extremely friable, crenulated micaceous deposits, although difficult to prepare, are of particular interest. Sections cut normal to the lamination and microfold axes show both the rock and the individual mica flakes to have suffered very considerable contortion. Areas occur in which sub-parallel kink planes bound kink bands which are symmetrically orientated to them. These are more frequently towards the central portions of sectioned mica-rich beds (Fig. 2). The kink planes, although they sometimes diverge, are commonly orientated normal to the bedding planes (Plate 10, C). Paterson (1964, 30) has suggested that this type of structure represents an intense degree of deformation and severe shortening in a direction parallel to the rock lamination. In other domains of the rock, bundles of micaceous flakes have been deformed by bend gliding. The two deforming processes of kinking and bend gliding are frequently transitional into each other in a manner comparable to that originally displayed by Orowan (1942) in metal wire (plate 10,D). In some instances the micaceous minerals in the domain of bend gliding associated with a kink plane tend to form into conjugate folds (Plate 11, E). The intensity of deformation through the thickness of the micaceous beds is surprisingly variable. Large areas of the rock do not exhibit kink planes, but only display nearly geometrically symmetrical plications. In these areas, the micaceous minerals, acting as relatively strong, competent and yet elastic units, form microfolds which more generally approximate to a parallel or concentric pattern of folding. For this reason, deformation in the cuspate cores ofthe parallel microfolds is often complex (Plate 10,D). Conditions of intense deformation in the microfold cores in certain cases produce 'neck' structures (Hills, 1963, 244), where compression causes a break-out or core material by fracture (Plate 11, F). Microscopically there is little evidence for any shearing-out of the microfold limbs of the rock by processes which result in strain-slip cleavage (Turner & Weiss, 1963, 98). Signs of nco-crystallisation of the
PROC. GEOL. ASS., VOL. 78 (1967)
PLATE 9
A. Structural plications on a micaceous band, Cil-maenIIwyd quarry, L1andeilo (photograph by E. Martin, Esq.)
B. Separation of muscovite flakes showing birefringent grain boundaries (crossed nicols, x 38) [To face p. 280
PROC. GEOL. ASS., VOL. 78 (1967)
PLATE 10
C. Sub-parallel kink planes in the micaceous deposit (crossed nicols, x 65)
D. Transition of a kink plane into an area of bend gliding (ordinary light, x 54)
PROC. GEOL. ASS. , VOL . 78 ( 196 7)
PLATE 11
E. Dom ain of co nj uga te microfoldin g sit uat ed between regions o f norma l bend glidin g a nd kin kin g (or dina ry light , x 33)
F . Break -out o f core material in parall el microfold \0 form a 'n eck ' struc ture (o rdina ry light , x 45)
G. Det a il of several mu sco vite flak es, showing microscopic kin k plane passing into zone of bend gliding (o rd ina ry light , x 160)
DEFORMED MICACEOUS DEPOSITS
281
flaky minerals along incipient shear planes are similarly absent. Axial plane extension in the crenulations seems to have taken place mainly by the process of kinking. de Sitter (1956, 217), in reference to accordionfolding, has shown such extension develops when kink planes are present. The kink planes in the strongly micaceous and crenulated Long Quarry Bed sediments are also axial planes which bisect the kink folds. Dewey (1965, 461) recently considered this type of occurrence to be a special case of kinking only possible in reverse kink-zones. Rotation of the kink
~- E
< Fig. 2. Diagrammatic representation of narrow (e.g. half an inch) mica band showing possible occurrence of structures illustrated in Plates 9 to 11 (C to F respectively)
JOHN F. POTTER
282
a.
b.
c.
d.
Fig. 3. Microscopic distortion and fracture of mica flakes a. Bend gliding in a slightly deformed flake of muscovite showing the development of cleavage parting (approximate magnification x 48) b. Fracture and bend gliding of flakes (approximate magnification x 28) c. Sub-parallel kink planes and cleavage parting in a muscovite flake (approximate magnification x 26) d. Sub-parallel kink planes or fractures in a muscovite flake. The flake is splayed out at the crest of one kink plane (approximate magnification x 26)
bands occurs by slip along the layers and cleavage of the micaceous minerals. Individual mica flakes show comparable deformations to those seen in the rock as a whole (Fig. 3). Microscopic kink planes again pass into zones of bend gliding (Plate 11, G). Where the rock has been thrown into tightly folded corrugations, individual flakes may be fractured, or very rarely splayed out, at the points of greatest flexure. The degree of bending required to fracture a mica flake was extremely variable; presumably it was related to the confining forces and other factors that controlled the elasticity of each grain. However, flakes bent to form an angle of less than 75° are invariably fractured. Bend gliding in individual flakes or bundles of flakes, of the type exhibited in Fig. 3a, where the cleavage parting of the bent flakes is developed, naturally tends to thicken the crests and relatively attenuate the limbs of the microfolds. Crenulation in the rock does not appear to have been strongly influenced by the more equidimensional siliceous grains, even when these grains reach a comparatively large size; nor do these grains show any evidence of rotation independent of the surrounding rock matrix. Frequently pressuresolution has taken place between the grain boundaries of the siliceous and flaky minerals, both materials displaying signs of solution, but the former being more extensively affected (cf. Nicholson, 1966). 4. ANALYSIS OF STRUCTURES In a recent paper Williams (1961) illustrated that in instances of similar folding the more incompetent beds accommodate themselves into the
DEFORMED MICACEOUS DEPOSITS
283
interspaces by flow as a result of considerable changes in the shape of the fold cross-section during the development of the fold. Drag folds within the incompetent layers were accredited to the differential movement between the flow laminae in a direction sub-parallel to the axial plane of the principal fold structure. These drag folds are best observed in thin competent layers (Ramberg, 1963, 97) within the incompetent unit. They have an asymmetrical shape, and have long been recognised as important structural features. The associated regional structure has frequently been inferred from their attitude (as in Leith, 1923; Billings, 1958; and others). A
B.
Fig. 4. A. Folded competent (stippled) and incompetent beds (partly after Williams, 1961)
B. Folded competent beds, with inset of lenticular micaceous band. Arrows
indicate direction in which material tends to move
With parallel or concentric folding the beds undergo different confining forces and a different type of deformation (Fig. 4). Bedding plane slip must take place and flexural-slip folding of this type has often been cited (as Hills, 1963, 227). Normally, bedding plane slip would occur, as the name indicates, along the bedding planes of successive competent units rather than within a bed itself, for if an incompetent unit were involved similar folding would be produced. In the Long Quarry Beds the micaceous bands offer the rare example in which bedding plane slip has occurred within the unit. In this instance, there has been little variation in thickness of the confined accommodating unit during deformation. The mica flakes at the top and bottom of the micaceous bed are parallel to the bedding planes, and slip without evident contortion has taken place along their surfaces. More remote from the margins of the bed the mica flakes become wavy and at the centre of the micaceous layer crenulations are well developed. Geometrically these crenulations are almost symmetrical; they do not show the pronounced asymmetry that one would expect on the flanks of a layer fold structure (Wilson, 1961, 505, and fig. 37), and their axial planes are radially orientated with respect to the fold as a whole. It would appear that during the formation of the monoclinal Towy anticline, micaceous deposits interbedded with competent sandstones in
2i4
JOHN F. POTTER
the Long Quarry Beds acted as units in which bedding plane slip took place. The resulting movement created irrespective of orientation, symmetrical or nearly symmetrical, crenulations on the bedding plane laminations of the micaceous beds. The axial planes of the crenulations, seen in the steeply dipping southern limb of the anticline, are radial to the anticline fold axis. The microfold axes in all the plicated micaceous beds examined were approximately horizontal. This would imply that the major structural feature in the vicinity of Llandeilo, and in particular the southern limb of the Towy anticline, is unlikely to be affected by more than a few degrees of plunge. Any small angle of plunge that does affect the fold limb is towards the west. There appears to be no direct relationship between the size of the microfolds in the rock and the lateral dimensions of the individual micaceous minerals; nor does it seem likely that included siliceous grains in the deposit created irregularities in the laminar flow lines to produce the crenulations. It would seem possible that the first production of microfolds or plications might result from the tendency to develop a wave surface at the boundary between the flaky materials moving relatively in opposite directions (Lamb, 1932, 373 et seq.; Biot, 1963). The production of experimental folding in rocks (Paterson & Weiss, 1962; Paterson, 1964) clearly shows that comparable structures can be produced by compressing schistose rocks in a direction approximately parallel to their schistosity. In the Long Quarry Beds a deforming process such as this latter would have been assisted by the lenticular form of the micaceous bands. Biot (1964, 1965) has recently presented a theoretical study of the type of deformation seen in the Long Quarry Bed micaceous bands, where multilayers are internally buckled under rigid confinement. He determined that the dominant wavelength of the mica crenulation should be about .fed = 1.90 ~hI H
where, hI is the distance between the folding slip planes within the mica and H is the total thickness of the confined micaceous layer.
Both .fed 2 and H2 can be directly measured so that it is feasible to calculate a mean value for hi, For the example at Cil-maen-llwyd quarry cited in the present text, where H = 36 mm.,.fed = 4.3 mm.; from which it is possible to determine that hi is approximately 0.14 rom. The approximate mean value of hi for those field measurements illustrated in Fig. 5 (Table II) is 0.26 rom. These figures in excess of the average thickness of the mica 2 The observed values for these in the crenulated micaceous bands will be slightly dissimilar to those in Biot's equation which is intended to be more applicable to the earlier phases of deformation. The creation of kink folds would especially augment this difference.
285
DEFORMED MICACEOUS DEPOSITS
TABLE II. Observed mean values of ~d, andcalculated values of ~H and hr for the various Long Quarry Bed micaceous bands listed in Table I
Locality (see Table I) I 2 2 3 4 4 4 5 5 5 5 5 5 6 6 6
Mean ~d mm. 4.3 1.75 1.5 2.5 4.0 4.0 10.5 8.0 1.25 1.75 2.5 4.5 1.25 2.0 1.5 2.0
lii
Approx. H
mm.
( hl=
6.0 3.4 1.7 3.0 4.8 4.2 6.9 5.6 1.4 2.6 2.6 4.1 1.7 2.2 1.4 1.9
36.0 11.6 3.0 9.0 23.0 18.0 47.0 31.0 2.0 7.0 7.0 17.0 3.0 5.0 2.0 3.5
~d
)
1.90r:m~H
2
0.14 0.D7 0.21 0.19 0.19 0.25 0.64 0.56 0.22 0.13 0.26 0.33 0.15 0.23 0.32 0.31 Mean hI = 0.26
12
11 X 10
q X
Mean
Ld
b
mm.
x x
.i o
x
x
x x x x x
3
F~
Mean Fig. 5. Graph of observed mean values of ~d plotted against the mean values of ~ii for tile various Long Quarry Bed micaceous bands listed in Table 1
286
JOHN F. POTTER
flakes and suggests that movement has not taken place between all the flakes present. In the micaceous layers in the Long Quarry Beds hi is variable and difficult to determine. It may represent the distance between cleavage partings, or the thickness of individual flakes or stacks of flakes. Presumably the properties of the micas are sufficiently similar in all the crenulated bands to conclude that approximately
.?d ex: {if This conclusion appears to be confirmed by field observation and measurement (Fig. 5). ACKNOWLEDGMENTS The author wishes to thank Dr. M. A. Biot, Dr. D. J. Maull, Prof. J. G. Ramsay and Dr. E. Williams for their advice and discussion. Thanks are also due to Dr. Gilbert Wilson who was kind enough to read and criticise a draft of the manuscript.
REFERENCES BILLINGS, M. P. 1958. Structural Geology. New Jersey. BlOT, M. A. 1963. Internal buckling under initial stress in finite elasticity. Proc, R. Soc. A, 273, 306-28. - - - . 1964. Theory of internal buckling of a confined multilayered structure. Bull. geol. Soc. Am., 75, 563-8. - - - . 1965. Further development of the theory of internal buckling of multilayers. Bull. geol. Soc. Am., 76, 833-40. DEWEY, J. F. 1965. Nature and Origin of Kink-bands. Tectonophysics, 1,459-94. HILLS, E. S. 1963. Elements of Structural Geology. London. KRYNINE, P. D. 1940. Petrology and Genesis of the Third Bradford Sand. Pennsylvania State College Bulletin, 29, 82. LAMB, H. 1932. Hydrodynamics. London. LEITH, C. K. 1923. Structural Geology. New York. NICHOLSON, R. 1966. Metamorphic Differentiation in Crenulated Schists. Nature, Lond. 209, 68-9. PATERSON, M. S. 1964. Experimental Deformation of Rock. Discovery; 25, 27-31. - - - . & L. E. WEISS, 1962. Experimental folding in rocks. Nature, Lond. 195, 1046-8. POTTER, J. F. & J. H. PRICE, 1965. Comparative Sections through Rocks of Ludlovian -Downtonian Age in the Llandovery and Llandeilo Districts. Proc, Geol. Ass., 76, 372-404. OROWAN, E. 1942. A Type of Plastic Deformation New in Metals. Nature, Land. 149, 643-4. RAMBERG, H. 1963. Evolution of Drag Folds. Geol. Mag., 100, 97-106. de SITTER, L. U. 1956. Structural Geology, l st ed. London. STRAHAN, A., T. C. CANTRILL, E. E. L. DIXON & H. H. THOMAS, 1907. The Geology of the South Wales Coal-field. Part VII. The Country around Ammanford.
Mem, Geol. Surv, U.K. TURNER, F. J. & L. E. WEISS, 1963. Structural Analysis of Metamorphic Tectonites. New York.
DEFORMED MICACEOUS DEPOSITS
E. 1961. The Deformation of Confined, Incompetent Layers in Folding. Geol. Mag., 98, 317-23. G. 1961. The Tectonic Significance of Small Scale Structures, and their Importance to the Geologist in the Field. Annis. Soc. geol. Belg., 84, 423-548.
WILLIAMS, WILSON,
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J. F. Potter Norwood Technical College Knight's Hill London S.E.27