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Limestone diagenesis during deep burial and its relationship to deformation mechanisms of calcite
Sedimentary Geology, 35 (1983) 153-157 Elsevier Science Publishers B.V., Amsterdam
153 Printed in The Netherlands
Discussion
L I M E S T O N E DIAGENESIS D U R I N G DEEP BURIAL A N D ITS RELATIONSHIP TO D E F O R M A T I O N MECHANISMS OF CALCITE
J. D I X O N and V.P. W R I G H T
Department of Geology, University College, Cardiff CF1 1XL ( U.K.) Department of Earth Sciences, Open University, Milton Keynes (U.K.) (Received November 20, 1981 ; accepted December 30, 1982)
INTRODUCTION
Borak and Friedman (1981) in a recent paper in this journal described deformation of limestones in deep boreholes from the Anadarko Basin, Oklahoma. The processes operating in the deep sub-surface are little understood and such a study as that of Borak and Friedman is important, but several conclusions drawn by those authors require further comment. The aim of this paper is to clarify several points made by those authors as regards the significance of the fabrics they describe and the deformation mechanisms operating at depth especially the roles of cataclastic and plastic processes. C A L C I T E D E F O R M A T I O N FABRICS
Borak and Friedman document several fabrics which they consider (p. 144) not to have previously described from unmetamorphosed limestomes. The formation of micron-sized grains along twin lamellae which they describe was documented by Voll (1960) and Wardlaw (1962), and these fabrics are reviewed by Bathurst (1975). Borak and Friedman (p. 147) also describe degrading recrystallization of the borders of large calcite crystals (e.g. echinoderm fragments). This fabric has also been documented by Voll (1960) in his classic paper on petrography, and Tucker and Kendall (1973) have also described this fabric from low-grade metamorphic limestones from Germany. Whilst Borak and Friedman have recognised a close similarity between the Hunton Carbonates and deformed marble, particularly by showing Fig. 12b (p. 146), it seems that they have chosen to ignore the possibility of similar mechanisms occuring during the development of the fabrics in the Hunton Carbonates. The twinning and growth of twins in calcite crystals is closely related to the formation of dislocations and to the glide of dislocations within slip planes (Nicolas and Poirier, 1976). This has the effect of producing glide plane twins on (1011) with (1012) as 0037-0738/83/$03.00
© 1983 Elsevier Science Publishers B.V.
154
the glide direction. According to Barber and Wenk (1976) this is favoured by high temperatures and could be expected during metamorphism, however this need not exclude the twins from unmetamorphosed rock providing the conditions of low strain rates can be maintained for geologically long periods of time (Spencer, 1969; White, 1977). At whatever temperature the twins form they are a result of intracrystalline plasticity of the calcite; the calcite does not behave as a brittle solid. Further twinning common during the deformation of calcite results from gliding on (0112) with C0111) as the glide direction. These bisect the acute angle of the rhombs of the calcite, that is, those twins formed on (1011) and are again the result of intracrystalline plasticity of the calcite. Twinning on (0112) is common at low temperatures. Further features which would indicate the plastic behaviour of calcite are undulose extinction and bent cleavages, both features indicating crystal lattice defects. Returning to the work of Borak and Friedman, and applying these arguments of plastic deformation, shows that the multiple twinning cut by later twins (Fig. 12a) are a result of primary and secondary twinning in suitable directions. This is a feature of plastic intracrystalline slip and is in no way related to "breakage" which would suggest cataclasis and brittle behaviour. Similarly the bent cleavages figured in fig. 13 are the result of crystal lattice defects of the calcite and not to its behaviour as a brittle solid. In this context something should also be said about the "granulation" of large calcite grains. As stated above the calcite has been subjected to plastic deformation and has twinned as a direct result of this. Twinning and bending of cleavages, together with undulose extinction, are the visible effects of strain hardening (Hayden et al., 1965; Nicolas and Poirier, 1976), that is, the calcite resist further deformation by the production of these features. To allow a continuous deformation, therefore, it is essential to produce new areas within the calcite (or limestone) capable of undergoing further deformation. This is accomplished by dynamic recovery a n d / o r dynamic recrystallization (White, 1976). The effect of both is to reduce the features of strain hardening and allow further deformation. Again the processes are governed by the plasticity of calcite and are not a result of cataclastic granulation. Figure 12b of Borak and Friedman, from a deformed marble and used as a comparison, would appear to show dynamic recrystallization of calcite as a result of plastic deformation. As this fabric occurs because of plastic deformation the Hunton Carbonates may also represent these features. However, if the Hunton Carbonates are thought of as cataclastic then no similarity exists with the deformed marble. The texture presented in fig. 15 (p. 148) does not show conclusive evidence for either the cataclastic or plastic mode of deformation, though it is clear from fig. 15 that the rock has been subjected to plastic deformation as evidenced by the calcite twins. The fact that some of the smaller crystals also shows what appears to be twinning suggests the possibility that the plastic deformation of calcite has been continuing whilst the grain-size reduction has proceeded. This would seem a strong possibility especially as Borak and Friedman have recognised a strong increase in
155
undulose quartz. Undulose quartz results from plastic intracrystalline deformation but requires higher strain rates and temperatures relative to calcite. Therefore, if conditions were suitable for plastic deformation of quartz it is likely that they were suitable for plastic deformation of calcite. We are examining the effects of plastic deformation of calcite at the moment and it is becoming clear that it is strongly dependent on grain size. This would account for the fact that these features of plastic deformation are observed only in large grains such as echinoderm fragments (Voll, 1960; Bathurst, 1975). Features of pressure solution dominate at small grain sizes but the greater the grain size the greater the likelihood of plastic deformation. We would, however, concur with Borak and Friedman on the fact that metamorphic conditions are not essential for these features to be developed, but would differ strongly in that we consider the features to occur primarily as a result of plastic deformation (Dixon and Wright, in press). REFERENCES Barber, D.J. and Wenk, H.R., 1976. Defects in calcite in carbonate rocks. In: H.R. Wenk (Editor), Electron Microscopy inMineralogy. Springer, Berlin, pp. 429-442. Barber, D.J. and Wenk, H.R., 1979. On geological aspects of calcite microstructures. Tectonophysiccs. 54:45-60. Bathurst, R.G.C., 1975. Carbonate Sediments and their Diagenesis. Elsevier, Amsterdam, 660 pp. Borak, B. and Friedman, G.M., 1981. Textures of sandstones and carbonate rocks in the world's deepest wells (in excess of 30,000 ft or 9.1 km): Anadarko Basin, Oklahoma. Sediment. Geol., 29:133-152. Dixon, J. and Wright, V.P., in press. The origin of crystal diminution in some limestones from South Wales. Sedimentology. Hayden, H.W., Moffatt, W.G. and Wulff, J., 1965. The Structure and Properties of Materials. Volume III. Mechanical Behaviour. Wiley, New York, N.Y. Nicolas, A. and Poirier, J.P., 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. Wiley, London, 437 pp. Spencer, E.W., 1969. Introduction to the Structure of the Earth. McGraw-Hill, New York, N.Y., 597 pp. Tucker, M.E. and Kendall, A.C., 1973. The diagenesis and low grade metamorphism of Devonian styliolinid-rich pelagic carbonates from West-Germany: possible analogues of recent pteropod oozes. J. Sediment. Petrol., 43: 672-687. Voll, G., 1960. New York on Petrofabrics. Geol. J., 2:503-567. Wardlaw, N.C., 1962. Aspects of diagenesis in some Irish Carboniferous limestones. J. Sediment. Petrol., 32:776-780. White, S., 1976. The effects of strain on the microstructures, fabrics and deformation mechanisms in quartzites. Philos. Trans. R. Soc. London. Ser. A, 283:69-86. White, S., 1977. Geological significance of recovery and recrystallization processes in quartz. Tectonophysics, 39:143-170.
153 Printed in The Netherlands
Discussion
L I M E S T O N E DIAGENESIS D U R I N G DEEP BURIAL A N D ITS RELATIONSHIP TO D E F O R M A T I O N MECHANISMS OF CALCITE
J. D I X O N and V.P. W R I G H T
Department of Geology, University College, Cardiff CF1 1XL ( U.K.) Department of Earth Sciences, Open University, Milton Keynes (U.K.) (Received November 20, 1981 ; accepted December 30, 1982)
INTRODUCTION
Borak and Friedman (1981) in a recent paper in this journal described deformation of limestones in deep boreholes from the Anadarko Basin, Oklahoma. The processes operating in the deep sub-surface are little understood and such a study as that of Borak and Friedman is important, but several conclusions drawn by those authors require further comment. The aim of this paper is to clarify several points made by those authors as regards the significance of the fabrics they describe and the deformation mechanisms operating at depth especially the roles of cataclastic and plastic processes. C A L C I T E D E F O R M A T I O N FABRICS
Borak and Friedman document several fabrics which they consider (p. 144) not to have previously described from unmetamorphosed limestomes. The formation of micron-sized grains along twin lamellae which they describe was documented by Voll (1960) and Wardlaw (1962), and these fabrics are reviewed by Bathurst (1975). Borak and Friedman (p. 147) also describe degrading recrystallization of the borders of large calcite crystals (e.g. echinoderm fragments). This fabric has also been documented by Voll (1960) in his classic paper on petrography, and Tucker and Kendall (1973) have also described this fabric from low-grade metamorphic limestones from Germany. Whilst Borak and Friedman have recognised a close similarity between the Hunton Carbonates and deformed marble, particularly by showing Fig. 12b (p. 146), it seems that they have chosen to ignore the possibility of similar mechanisms occuring during the development of the fabrics in the Hunton Carbonates. The twinning and growth of twins in calcite crystals is closely related to the formation of dislocations and to the glide of dislocations within slip planes (Nicolas and Poirier, 1976). This has the effect of producing glide plane twins on (1011) with (1012) as 0037-0738/83/$03.00
© 1983 Elsevier Science Publishers B.V.
154
the glide direction. According to Barber and Wenk (1976) this is favoured by high temperatures and could be expected during metamorphism, however this need not exclude the twins from unmetamorphosed rock providing the conditions of low strain rates can be maintained for geologically long periods of time (Spencer, 1969; White, 1977). At whatever temperature the twins form they are a result of intracrystalline plasticity of the calcite; the calcite does not behave as a brittle solid. Further twinning common during the deformation of calcite results from gliding on (0112) with C0111) as the glide direction. These bisect the acute angle of the rhombs of the calcite, that is, those twins formed on (1011) and are again the result of intracrystalline plasticity of the calcite. Twinning on (0112) is common at low temperatures. Further features which would indicate the plastic behaviour of calcite are undulose extinction and bent cleavages, both features indicating crystal lattice defects. Returning to the work of Borak and Friedman, and applying these arguments of plastic deformation, shows that the multiple twinning cut by later twins (Fig. 12a) are a result of primary and secondary twinning in suitable directions. This is a feature of plastic intracrystalline slip and is in no way related to "breakage" which would suggest cataclasis and brittle behaviour. Similarly the bent cleavages figured in fig. 13 are the result of crystal lattice defects of the calcite and not to its behaviour as a brittle solid. In this context something should also be said about the "granulation" of large calcite grains. As stated above the calcite has been subjected to plastic deformation and has twinned as a direct result of this. Twinning and bending of cleavages, together with undulose extinction, are the visible effects of strain hardening (Hayden et al., 1965; Nicolas and Poirier, 1976), that is, the calcite resist further deformation by the production of these features. To allow a continuous deformation, therefore, it is essential to produce new areas within the calcite (or limestone) capable of undergoing further deformation. This is accomplished by dynamic recovery a n d / o r dynamic recrystallization (White, 1976). The effect of both is to reduce the features of strain hardening and allow further deformation. Again the processes are governed by the plasticity of calcite and are not a result of cataclastic granulation. Figure 12b of Borak and Friedman, from a deformed marble and used as a comparison, would appear to show dynamic recrystallization of calcite as a result of plastic deformation. As this fabric occurs because of plastic deformation the Hunton Carbonates may also represent these features. However, if the Hunton Carbonates are thought of as cataclastic then no similarity exists with the deformed marble. The texture presented in fig. 15 (p. 148) does not show conclusive evidence for either the cataclastic or plastic mode of deformation, though it is clear from fig. 15 that the rock has been subjected to plastic deformation as evidenced by the calcite twins. The fact that some of the smaller crystals also shows what appears to be twinning suggests the possibility that the plastic deformation of calcite has been continuing whilst the grain-size reduction has proceeded. This would seem a strong possibility especially as Borak and Friedman have recognised a strong increase in
155
undulose quartz. Undulose quartz results from plastic intracrystalline deformation but requires higher strain rates and temperatures relative to calcite. Therefore, if conditions were suitable for plastic deformation of quartz it is likely that they were suitable for plastic deformation of calcite. We are examining the effects of plastic deformation of calcite at the moment and it is becoming clear that it is strongly dependent on grain size. This would account for the fact that these features of plastic deformation are observed only in large grains such as echinoderm fragments (Voll, 1960; Bathurst, 1975). Features of pressure solution dominate at small grain sizes but the greater the grain size the greater the likelihood of plastic deformation. We would, however, concur with Borak and Friedman on the fact that metamorphic conditions are not essential for these features to be developed, but would differ strongly in that we consider the features to occur primarily as a result of plastic deformation (Dixon and Wright, in press). REFERENCES Barber, D.J. and Wenk, H.R., 1976. Defects in calcite in carbonate rocks. In: H.R. Wenk (Editor), Electron Microscopy inMineralogy. Springer, Berlin, pp. 429-442. Barber, D.J. and Wenk, H.R., 1979. On geological aspects of calcite microstructures. Tectonophysiccs. 54:45-60. Bathurst, R.G.C., 1975. Carbonate Sediments and their Diagenesis. Elsevier, Amsterdam, 660 pp. Borak, B. and Friedman, G.M., 1981. Textures of sandstones and carbonate rocks in the world's deepest wells (in excess of 30,000 ft or 9.1 km): Anadarko Basin, Oklahoma. Sediment. Geol., 29:133-152. Dixon, J. and Wright, V.P., in press. The origin of crystal diminution in some limestones from South Wales. Sedimentology. Hayden, H.W., Moffatt, W.G. and Wulff, J., 1965. The Structure and Properties of Materials. Volume III. Mechanical Behaviour. Wiley, New York, N.Y. Nicolas, A. and Poirier, J.P., 1976. Crystalline Plasticity and Solid State Flow in Metamorphic Rocks. Wiley, London, 437 pp. Spencer, E.W., 1969. Introduction to the Structure of the Earth. McGraw-Hill, New York, N.Y., 597 pp. Tucker, M.E. and Kendall, A.C., 1973. The diagenesis and low grade metamorphism of Devonian styliolinid-rich pelagic carbonates from West-Germany: possible analogues of recent pteropod oozes. J. Sediment. Petrol., 43: 672-687. Voll, G., 1960. New York on Petrofabrics. Geol. J., 2:503-567. Wardlaw, N.C., 1962. Aspects of diagenesis in some Irish Carboniferous limestones. J. Sediment. Petrol., 32:776-780. White, S., 1976. The effects of strain on the microstructures, fabrics and deformation mechanisms in quartzites. Philos. Trans. R. Soc. London. Ser. A, 283:69-86. White, S., 1977. Geological significance of recovery and recrystallization processes in quartz. Tectonophysics, 39:143-170.