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New techniques for detecting sedimentary fabrics in evaporite rocks
Sedimentary Geology, 72 (1991) 147-155
147
Elsevier Science Publishers B.V., Amsterdam
Technical Note
New techniques for detecting sedimentary fabrics in evaporite rocks Rolf Langbein and Volker Schmidt University of Greifswald, Department of Geology, JahnstraBe 17a, 0-2200 Greifswald, F R. G.
Received November 21, 1990; revised version accepted April 16, 1991
ABSTRACT Langbein, R. and Schmidt, V., 1991. New techniques for detecting sedimentary fabrics in evaporite rocks. Sediment. Geol., 72: 147-155. Three new methods which are used for demonstrating sedimentary and diagenetic features in evaporite rocks--anhydrite, halite and sylvite--include: (a) the acetate peel technique after etching with NaCl-solution; (b) the scattered-light photomicrograph technique with thin-sections; (c) the epoxy peel technique with imprints of a vapour-etched polished slab. The examples given demonstrate that when combining these methods, it is possible to recognize all kinds of features which result from differences in crystal- and grain-size and in grain-shape, as well as by inclusions of brine or organic matter.
Introduction For the last few years, evaporites have increasingly been regarded not merely as chemical precipitates, but also as a normal sediment in their own right. This new approach needs new methods of investigation. Unfortunately, in m a n y cases the normal petrographic techniques employing the polarising microscope and thin-sections are not effective because of the high birefringence of anhydrite and the optical isotropy of halite and sylvite. Both, staining techniques and acetate peel techniques, which are commonly used for detecting sedimentary features in carbonate rocks, can also be applied with some modifications to evaporites (H. Gresner, personal communication, 1980; Larsen and Lagoni, 1984; Ney, 1986). Limestones can further be investigated by using fluorescence microscopy (Dravis and Yurewicz, 1985), or the white card method (Delgardo, 1977; Zenger, 1979; Folk, 1987). These methods involve three main principles: the formation of coloured 0037-0738/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
cation-organo complex ions; the solution anisotropy caused by different minerals or grain sizes; and reflection of inclusions in diffuse or obliquely reflected light. Being aware of these facts, some new methods have been developed, and tested, in order to make the sedimentary structures in evaporites visible and to distinguish them from diagenetic ones.
Acetate peel technique This method is similar to that employed in carbonate rocks. Thus, polished slabs of rocks are etched by HC1 (concentrated, during a time of some minutes) and the etched surface particles are fixed and transferred with the help of aceton on an acetate peel. This technique can be used for anhydrite or gypsum rocks with a high content of dolomite. If evaporites free of carbonate are to be investigated, the polished slabs can be leached by a solution of 10% NaC1 in distilled water, or in the
R. LANGBEINAND V. SCHMIDT
148 TABLE 1 Summary of different peel techniques Rock type
Leaching agent
Leaching time
Fixing method
Carbonatic anhydrite Massive anhydrite Halitic anhydrite Halite Halitic sylvinite
HCI, concentrated NaCl-solution (10%) Aqua dest. Water vapour KCl-sohition, nearly saturated KC1-MgC12-solution, nearly saturated
minutes 24 hours minutes 1 minute (dried with air) 1 minute (dried with ethyl-alcohol) 1 / 2 hour (dried with acetone)
Acetate peel Acetate peel Acetate peel Epoxy imprint Epoxy imprint
Halitic carnallitite
Epoxy imprint
Fig. 1. Gypsum crystal-grass (sedentite), pseudomorphosed by anhydrite; the pore fillings consist of halite. Acetate peel from an etched rock slab.
NEW TECHNIQUES FOR DETECTING SEDIMENTARY FABRICS IN EVAPORITE ROCKS
] 49
Fig. 2. Algal mat sediment (dark grey), anhydrite pseudomorphs after gypsum, cemented by isometric anhydrite (white). Scattered-light photogram from a thin-section.
Fig. 3. Algal mat sediment, anhydrite pseudomorphs after gypsum; only two layers are cemented (light gray). Scattered light-photogram from a thin-section.
]50
case of sulphate-chloride intergrowth, by pure water (during some minutes). After treatment with the former solution for 24 h, good results can be obtained especially concerning pure anhydrite rocks (Fig. 1).
Scattered-light photo technique Inclusions of organic matter and brines in evaporite rocks can be studied in detail with thin slides using obliquely reflected illumination with a white card (Folk, 1987). Since many samples of
R. LANGBE1N A N D V. S C H M I D T
anhydrite contain organic inclusions due to their microbial origin and rock salts often contain brine inclusions marking zonal growth, this technique may also be applied to many evaporite rocks. However, better results can be achieved by using the appropriate thin-section as the object of a photographic enlargement on a plane film (Figs. 2-6). In this case a covered thin-section is used instead of a film negative in a magnifying camera. Thereby it is necessary to defocus the source of light in order to achieve an indirect illumination of the thin-section. The fine inclusions in the rock
Fig. 4. Anhydrite suberosional breccia, cemented and recrystallized by isometric and idiomorphic anhydrite. Scattered-light photogram from a thin-section.
NEW TECHNIQUES
FOR DETECTING
SEDIMENTARY
F A B R I C S IN E V A P O R I T E
slide reflect the oblique light and a scattered-light negative can be obtained instead of a photo. The films prepared in this way are of a high contrast and may serve as negatives for further photographic work. They allow a high magnification of details as well as an overview, especially when large slides (5 x 5 cm) are used. The scattered-light film technique is suitable for anhydrite rocks with primary growth features (sensu stricto) or sedentary (that means bottom-grown features as crystal grass or algal biostromes, sedentites) ones, but not for nodular anhydrites replacing carbonates or clayey siliciclastic soils (sabkha anhydrites with chicken-wire structures). Diagenetic alterations as for example compaction, brecciation and recrystallisation, can be clearly detected. The method may also be used for anhydrite rocks with great differences in grain size and for rock salt with m a n y inclusions of relic brines.
Epoxy peel technique The epoxy peel technique is actually a special variety of the acetate peel technique which can be
ROCKS
151
used for chloride evaporites (Figs. 7 and 8). In salt rocks and sylvinites, the detection of sedimentary and diagenetic features is very difficult because of the optical isotropy of the minerals which make investigation with the polarisation microscope impossible. Additionally, the crystal-size of these rocks is very large so that even a large thin-section is often not large enough to detect sedimentary structures. Therefore, a distinct progress can be achieved by applying the peel method to chloride rocks. The surface of a polished rock slab is carefully leached with water vapour. The best procedure is to freeze the slab and bring it then to the open air. A careful solvation produces a microrelief on the surface. This is valid for monomineral rocks. In polymineral rocks, like sylvitic halitites or carnallitic halitites, the relief is more marked because of the differences in the solubility of the salt minerals. The microrelief can be filled with a liquid preparation of an epoxy resin which is fixed after the hardening of the resin. We use a twocomponent resin and work at a temperature of 50°C to yield a low-viscosity liquid. The epoxy peel may be stabilized by glass slide or by an
Fig. 5. Anhydrite cement of a breccia (same as Fig. 4). Scattered-light photogram from a thin-section.
152
acetate peel and then the rock slab can be leached away. The epoxy peel does not only mark the outlines of the grains and their size but also the orientation of the grains. This is due to the fact that a careful solvation of the polished crystal surface produces anisotropic solution effects of the same types as " H A U Y ' s decrescences", i.e. a system of minute cubes which are parallel to the orientation of the lattice. The epoxy peel reflects the incident light depending on the orientation of
R. L A N ( J B E I N A N D V. S C H M I D T
the crystal lattice. Thus a photographic magnification of the peel produced by defocused light resuits in a high-contrast picture with all intensity transitions from white to grey and black. Such film enlargements are suitable for a computeraided mechanical grain-size analysis. Even the peel itself can be used for orientation measurements by means of a goniometer. Provided that the epoxy peel has been carefully prepared, it can be used for investigations within
Fig. 6. Algal mat sediment. The sedimentary particles (dark grey) consist of anhydrite pseudomorphs after gypsum, and they are cemented by fibrous anhydrite cement A (white, because of inclusions) and halite cement C (black, because free of inclusions). Scattered-light photogram from a thin-section.
NEW T E C H N I Q U E S F O R D E T E C T I N G S E D I M E N T A R Y FABRICS IN EVAPORITE ROCKS
153
Fig. 7. Clastic halite sediment, showing bedding resulting from alteration of fine- and coarse-grainedhalite crystals. Epoxy peel from a vapour-etchedrock slab. the macroscopic range, with the microscope using oblique light as well as with the scanning electron microscope. We prepared pictures with an 8000fold magnification. With this technique it is possible to study the boundaries in extremely fine detail. Another advantage of this new method is that most of the work can be done in the mine itself or at the site of the drilling well.
Examples Due to the varying content of bituminous inclusions it is possible to study the details of sedi-
mentation and diagenesis in the sedimentary anhydrite rocks which have not been altered intensively. Figure 2 for instance shows that the rock consists of loose sedimentary accumulations of algal mats and an amount of fibrous cement A (cementite, according to the terminology introduced by Langbein, 1987). Figure 3, however, shows an example with a low percentage of cement, i.e. where the algal mud had already been compacted before lithification (compactite: Langbein, 1987). Therefore, the details are almost destroyed. Provided there are clear differences in grain
154
R. L A N G B E I N A N D V. S C H M I D T
Fig. 8. Deformed halite rock. The halite crystals show a typical foliation (schistosity) and deformation of a fine-grained layer. Epoxy peel from an etched rock slab.
size, shape and degree of contamination, the postgenetic overprinting and deformation of anhydrite rocks can easily be traced back by using the new technique. In Figs. 4 and 5 it becomes obvious in which way the isometric, idiomorphic, corrotopic (a term introduced by Machel, 1985: corrosionalidiomorphic) anhydrite cements the anhydrite breccia and in which way it partially corrodes the fine-grained xenomorphic anhydrite. This recrystallization m a y also be studied in detail with the scanning electron microscope. In fine-grained rock salt, the lamination due to grain-size differences can be well investigated.
Thus it becomes possible to detect longer breaks of sedimentation connected with recrystallisation (grain coarsening) of the sediment surface as well as the formation of relief at the bottom which is caused by dissolution (Fig. 7). In the polymineral evaporites, the age relations of the components can be examined. As far as deformed rock salt layers are concerned, it can be identified whether the rocks were d~;~rmed eodiagenetically to synsedimentary or whether deformation took place later under the influence of halokinetic movements. In the case of Fig. 8, halite crystals are clearly stretched in the fold axial plane and there-
NEW TECHNIQUES FOR DETECTING SEDIMENTARY FABRICS IN EVAPORITE ROCKS
fore show a foliation, Here the deformation place
later,
with
an
oriented
growth
took
occurring
vertically to the direction of pressure.
References Delgardo, F., 1977. Primary textures in dolostones and recrystallized lime stone: a technique for their microscopic study. J. Sediment. Petrol., 47: 1339-1341. Dravis, J.J. and Yurewicz, D.A., 1985. Enhanced carbonate petrography using fluorescence microscopy. J. Sediment. Petrol., 55: 795-804. Folk, L.R., 1987. Detection of organic matter in thin-sections of carbonate rocks using a white card. Sediment. Geol., 54: 193-200.
155
Langbein, R., 1987. The Zechstein Sulphates: the state of the art. Lect. Notes Earth Sci., 10:143-188 Larsen, J.G. and Lagoni, P., 1984. Fabric Analysis of Domal Rock Salt. Zechstein Salt Denmark, Proj. EFP-81, DGU Geol. Surv. Denmark, Copenhagen, 100 pp. Machel, H.G., 1985. Facies and Diagenesis of Upper Devonian Nisku Buildups in the Subsurface of Alberta. McGill Univ., Montreal, 392 pp. Ney, P., 1986. Gesteinsaufbereitung im Labor. Enke-Verlag, Stuttgart, 157 pp. Zenger, D.H., 1979. Primary textures in dolostones and recrystallized limestones: a technique for their microscopic study. J. Sediment. Petrol., 49: 677-678.
147
Elsevier Science Publishers B.V., Amsterdam
Technical Note
New techniques for detecting sedimentary fabrics in evaporite rocks Rolf Langbein and Volker Schmidt University of Greifswald, Department of Geology, JahnstraBe 17a, 0-2200 Greifswald, F R. G.
Received November 21, 1990; revised version accepted April 16, 1991
ABSTRACT Langbein, R. and Schmidt, V., 1991. New techniques for detecting sedimentary fabrics in evaporite rocks. Sediment. Geol., 72: 147-155. Three new methods which are used for demonstrating sedimentary and diagenetic features in evaporite rocks--anhydrite, halite and sylvite--include: (a) the acetate peel technique after etching with NaCl-solution; (b) the scattered-light photomicrograph technique with thin-sections; (c) the epoxy peel technique with imprints of a vapour-etched polished slab. The examples given demonstrate that when combining these methods, it is possible to recognize all kinds of features which result from differences in crystal- and grain-size and in grain-shape, as well as by inclusions of brine or organic matter.
Introduction For the last few years, evaporites have increasingly been regarded not merely as chemical precipitates, but also as a normal sediment in their own right. This new approach needs new methods of investigation. Unfortunately, in m a n y cases the normal petrographic techniques employing the polarising microscope and thin-sections are not effective because of the high birefringence of anhydrite and the optical isotropy of halite and sylvite. Both, staining techniques and acetate peel techniques, which are commonly used for detecting sedimentary features in carbonate rocks, can also be applied with some modifications to evaporites (H. Gresner, personal communication, 1980; Larsen and Lagoni, 1984; Ney, 1986). Limestones can further be investigated by using fluorescence microscopy (Dravis and Yurewicz, 1985), or the white card method (Delgardo, 1977; Zenger, 1979; Folk, 1987). These methods involve three main principles: the formation of coloured 0037-0738/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
cation-organo complex ions; the solution anisotropy caused by different minerals or grain sizes; and reflection of inclusions in diffuse or obliquely reflected light. Being aware of these facts, some new methods have been developed, and tested, in order to make the sedimentary structures in evaporites visible and to distinguish them from diagenetic ones.
Acetate peel technique This method is similar to that employed in carbonate rocks. Thus, polished slabs of rocks are etched by HC1 (concentrated, during a time of some minutes) and the etched surface particles are fixed and transferred with the help of aceton on an acetate peel. This technique can be used for anhydrite or gypsum rocks with a high content of dolomite. If evaporites free of carbonate are to be investigated, the polished slabs can be leached by a solution of 10% NaC1 in distilled water, or in the
R. LANGBEINAND V. SCHMIDT
148 TABLE 1 Summary of different peel techniques Rock type
Leaching agent
Leaching time
Fixing method
Carbonatic anhydrite Massive anhydrite Halitic anhydrite Halite Halitic sylvinite
HCI, concentrated NaCl-solution (10%) Aqua dest. Water vapour KCl-sohition, nearly saturated KC1-MgC12-solution, nearly saturated
minutes 24 hours minutes 1 minute (dried with air) 1 minute (dried with ethyl-alcohol) 1 / 2 hour (dried with acetone)
Acetate peel Acetate peel Acetate peel Epoxy imprint Epoxy imprint
Halitic carnallitite
Epoxy imprint
Fig. 1. Gypsum crystal-grass (sedentite), pseudomorphosed by anhydrite; the pore fillings consist of halite. Acetate peel from an etched rock slab.
NEW TECHNIQUES FOR DETECTING SEDIMENTARY FABRICS IN EVAPORITE ROCKS
] 49
Fig. 2. Algal mat sediment (dark grey), anhydrite pseudomorphs after gypsum, cemented by isometric anhydrite (white). Scattered-light photogram from a thin-section.
Fig. 3. Algal mat sediment, anhydrite pseudomorphs after gypsum; only two layers are cemented (light gray). Scattered light-photogram from a thin-section.
]50
case of sulphate-chloride intergrowth, by pure water (during some minutes). After treatment with the former solution for 24 h, good results can be obtained especially concerning pure anhydrite rocks (Fig. 1).
Scattered-light photo technique Inclusions of organic matter and brines in evaporite rocks can be studied in detail with thin slides using obliquely reflected illumination with a white card (Folk, 1987). Since many samples of
R. LANGBE1N A N D V. S C H M I D T
anhydrite contain organic inclusions due to their microbial origin and rock salts often contain brine inclusions marking zonal growth, this technique may also be applied to many evaporite rocks. However, better results can be achieved by using the appropriate thin-section as the object of a photographic enlargement on a plane film (Figs. 2-6). In this case a covered thin-section is used instead of a film negative in a magnifying camera. Thereby it is necessary to defocus the source of light in order to achieve an indirect illumination of the thin-section. The fine inclusions in the rock
Fig. 4. Anhydrite suberosional breccia, cemented and recrystallized by isometric and idiomorphic anhydrite. Scattered-light photogram from a thin-section.
NEW TECHNIQUES
FOR DETECTING
SEDIMENTARY
F A B R I C S IN E V A P O R I T E
slide reflect the oblique light and a scattered-light negative can be obtained instead of a photo. The films prepared in this way are of a high contrast and may serve as negatives for further photographic work. They allow a high magnification of details as well as an overview, especially when large slides (5 x 5 cm) are used. The scattered-light film technique is suitable for anhydrite rocks with primary growth features (sensu stricto) or sedentary (that means bottom-grown features as crystal grass or algal biostromes, sedentites) ones, but not for nodular anhydrites replacing carbonates or clayey siliciclastic soils (sabkha anhydrites with chicken-wire structures). Diagenetic alterations as for example compaction, brecciation and recrystallisation, can be clearly detected. The method may also be used for anhydrite rocks with great differences in grain size and for rock salt with m a n y inclusions of relic brines.
Epoxy peel technique The epoxy peel technique is actually a special variety of the acetate peel technique which can be
ROCKS
151
used for chloride evaporites (Figs. 7 and 8). In salt rocks and sylvinites, the detection of sedimentary and diagenetic features is very difficult because of the optical isotropy of the minerals which make investigation with the polarisation microscope impossible. Additionally, the crystal-size of these rocks is very large so that even a large thin-section is often not large enough to detect sedimentary structures. Therefore, a distinct progress can be achieved by applying the peel method to chloride rocks. The surface of a polished rock slab is carefully leached with water vapour. The best procedure is to freeze the slab and bring it then to the open air. A careful solvation produces a microrelief on the surface. This is valid for monomineral rocks. In polymineral rocks, like sylvitic halitites or carnallitic halitites, the relief is more marked because of the differences in the solubility of the salt minerals. The microrelief can be filled with a liquid preparation of an epoxy resin which is fixed after the hardening of the resin. We use a twocomponent resin and work at a temperature of 50°C to yield a low-viscosity liquid. The epoxy peel may be stabilized by glass slide or by an
Fig. 5. Anhydrite cement of a breccia (same as Fig. 4). Scattered-light photogram from a thin-section.
152
acetate peel and then the rock slab can be leached away. The epoxy peel does not only mark the outlines of the grains and their size but also the orientation of the grains. This is due to the fact that a careful solvation of the polished crystal surface produces anisotropic solution effects of the same types as " H A U Y ' s decrescences", i.e. a system of minute cubes which are parallel to the orientation of the lattice. The epoxy peel reflects the incident light depending on the orientation of
R. L A N ( J B E I N A N D V. S C H M I D T
the crystal lattice. Thus a photographic magnification of the peel produced by defocused light resuits in a high-contrast picture with all intensity transitions from white to grey and black. Such film enlargements are suitable for a computeraided mechanical grain-size analysis. Even the peel itself can be used for orientation measurements by means of a goniometer. Provided that the epoxy peel has been carefully prepared, it can be used for investigations within
Fig. 6. Algal mat sediment. The sedimentary particles (dark grey) consist of anhydrite pseudomorphs after gypsum, and they are cemented by fibrous anhydrite cement A (white, because of inclusions) and halite cement C (black, because free of inclusions). Scattered-light photogram from a thin-section.
NEW T E C H N I Q U E S F O R D E T E C T I N G S E D I M E N T A R Y FABRICS IN EVAPORITE ROCKS
153
Fig. 7. Clastic halite sediment, showing bedding resulting from alteration of fine- and coarse-grainedhalite crystals. Epoxy peel from a vapour-etchedrock slab. the macroscopic range, with the microscope using oblique light as well as with the scanning electron microscope. We prepared pictures with an 8000fold magnification. With this technique it is possible to study the boundaries in extremely fine detail. Another advantage of this new method is that most of the work can be done in the mine itself or at the site of the drilling well.
Examples Due to the varying content of bituminous inclusions it is possible to study the details of sedi-
mentation and diagenesis in the sedimentary anhydrite rocks which have not been altered intensively. Figure 2 for instance shows that the rock consists of loose sedimentary accumulations of algal mats and an amount of fibrous cement A (cementite, according to the terminology introduced by Langbein, 1987). Figure 3, however, shows an example with a low percentage of cement, i.e. where the algal mud had already been compacted before lithification (compactite: Langbein, 1987). Therefore, the details are almost destroyed. Provided there are clear differences in grain
154
R. L A N G B E I N A N D V. S C H M I D T
Fig. 8. Deformed halite rock. The halite crystals show a typical foliation (schistosity) and deformation of a fine-grained layer. Epoxy peel from an etched rock slab.
size, shape and degree of contamination, the postgenetic overprinting and deformation of anhydrite rocks can easily be traced back by using the new technique. In Figs. 4 and 5 it becomes obvious in which way the isometric, idiomorphic, corrotopic (a term introduced by Machel, 1985: corrosionalidiomorphic) anhydrite cements the anhydrite breccia and in which way it partially corrodes the fine-grained xenomorphic anhydrite. This recrystallization m a y also be studied in detail with the scanning electron microscope. In fine-grained rock salt, the lamination due to grain-size differences can be well investigated.
Thus it becomes possible to detect longer breaks of sedimentation connected with recrystallisation (grain coarsening) of the sediment surface as well as the formation of relief at the bottom which is caused by dissolution (Fig. 7). In the polymineral evaporites, the age relations of the components can be examined. As far as deformed rock salt layers are concerned, it can be identified whether the rocks were d~;~rmed eodiagenetically to synsedimentary or whether deformation took place later under the influence of halokinetic movements. In the case of Fig. 8, halite crystals are clearly stretched in the fold axial plane and there-
NEW TECHNIQUES FOR DETECTING SEDIMENTARY FABRICS IN EVAPORITE ROCKS
fore show a foliation, Here the deformation place
later,
with
an
oriented
growth
took
occurring
vertically to the direction of pressure.
References Delgardo, F., 1977. Primary textures in dolostones and recrystallized lime stone: a technique for their microscopic study. J. Sediment. Petrol., 47: 1339-1341. Dravis, J.J. and Yurewicz, D.A., 1985. Enhanced carbonate petrography using fluorescence microscopy. J. Sediment. Petrol., 55: 795-804. Folk, L.R., 1987. Detection of organic matter in thin-sections of carbonate rocks using a white card. Sediment. Geol., 54: 193-200.
155
Langbein, R., 1987. The Zechstein Sulphates: the state of the art. Lect. Notes Earth Sci., 10:143-188 Larsen, J.G. and Lagoni, P., 1984. Fabric Analysis of Domal Rock Salt. Zechstein Salt Denmark, Proj. EFP-81, DGU Geol. Surv. Denmark, Copenhagen, 100 pp. Machel, H.G., 1985. Facies and Diagenesis of Upper Devonian Nisku Buildups in the Subsurface of Alberta. McGill Univ., Montreal, 392 pp. Ney, P., 1986. Gesteinsaufbereitung im Labor. Enke-Verlag, Stuttgart, 157 pp. Zenger, D.H., 1979. Primary textures in dolostones and recrystallized limestones: a technique for their microscopic study. J. Sediment. Petrol., 49: 677-678.