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Origin and occurrence of glauconite in the green sandstone associated with unconformity, Bahariya Oases, Western Desert, Egypt
0731-72-t7/83 $3.00 + 0.00 Pergamon Press Ltd.
Journal ofAfrican Earth Sciences. Vol. I. No. 3/-t. pp. 321 t0325. 1983 Printed in Great Britain
Origin and occurrence of glauconite in the green sandstone associated with unconformity, Bahariya Oases, Western Desert, Egypt M. A.
KHALIFA
Menoufia University, Shibin El Kom, Egypt
(Received 13 October 1983) Abstract-This paper presents a new concept concerning the formation of authigenic glauconite in the unconformity surfaces. It is usually formed diagcnetically in subaerial and subcrop conditions on continental environments. In subaerial conditions glauconite selectively only replaces the massive and blocky calcite cements that formed in early stages of diagenesis. This may be attributed to the increase of the solubility of calcite at, and ncar, the weathered zone, under low temperature, pressure and pH values. On the other hand, in subcrop conditions glauconite prefers to replace the silica overgrowth around quartz grain without any replacement of the diagnetie zoned calcite cement. In such case, a reversible condition can be expected such as the increase of the solubility of silica under high temperature, pressure and pH values.
INTRODUCTION
OCCURRENCE OF GLAUCONITE
LITILE is known about the origin and occurrence of glauconite in the unconformity surface, except for the work of Goldman (1921) who recognized association of glauconite in the unconformity surface. Mesolella (1965), also found authigenic glauconite replacing nodules of collophane in the unconformity surface at the base of the Devonian Onondaga Limestone in New York State. The green sandstones are often encountered at and near the major unconformity that separates the Middle Cretaceous from Tertiary sediments in the Bahariya Oases. This is clearly in evidence at Gabel Radwan and Limestone Hill (Fig. 1). At Gabel Radwan (Fig. 2), the green sandstone separates the Lower Cenomanian Bahariya Formation (Said 1962) from the Oligocene Radwan Formation (Said and Issawi 1964). At this locality, the green sandstone has a deep green to bluish green, and is coarse (0.6 mm) to very coarse (1.2 mm) grained, poorly sorted and very hard. It attains a thickness of about one meter and extends laterally for several kilometers. At limestone Hill, the green sandstones are represented by two thin beds, each of which has an average thickness of about 45 cm. These beds rest unconformably on the Lower Cenomanian Bahariya Formation and conformably underlie the calcareous sandstones and limestones (Post Eocene?) that form the cape rock of the Limestone Hill (Fig. 2). The intention of this paper is to explain the causes of the restricted occurrence of glauconite in the unconformity surface. It calls attention to the possible generation of glauconite in the continental environments instead of marine conditions. Also, it possibly points out a new direction of work which could prove helpful in solving the problem of the physico-chemical condition needed for the glauconite formation.
Petrographic investigation of representative samples from Gabel Radwan show that. glauconite occurs as coatings, streaks and patches. Most of these glauconites have been produced entirely from the replacement of early deposited massive and blocky calcite cement. At the same time, glauconite does not replace quartz grains, where a sharp contact between them can be noticed (Fig. 3). Glauconite coatings may proceed at the contact between the calcite and quartz grains. In a further glauconitization process, glauconite encroaches and invades through the massive calcite along their cleavage giving the glauconite streaks (Fig. 4). Complete replacement of massive calcite by glauconite may produce glauconite patches (Fig. 3). At Limestone Hill, especially in the lower bed, glauconite selectively replaces the syntaxial silica overgrowth around quartz grains (Fig. 5), forming a glauconite coating. At the same time, glauconite does not show any textural replacement of the diagenetic zoned massive calcite cement, but it has been remarked that a new generation of unzoned calcite replaces the glauconite and even the quartz grains (Fig. 6). In the upper bed, glauconite commonly replaces the quartz grains in the form of irregular patches and coatings (Fig. 7). Most of these patches have been replaced by microspar and spray calcite; this is evidenced by the invasion of clear calcite patches within the dark green glauconite (Fig. 7).
ORIGIN AND DISCUSSION The most important factors used for the explanation of glauconite genesis in the present study, are the physico-chemical conditions (temperature, pressure and pH) under which glauconite prefers to replace calcite
321
M, A.
322
,
1f
KHALIFA
1,
o Ain EI Hiz
2d-__--J~---__#C-----_t_-----128
•
29
Fig. I, Location map of the studied area,
LIMESTONE HILL
G,RAOWAN T OT . T
'
r 'T , "T '
T 'T . T ' T 'T
' T ' T 'T
Green sandstones
'T '
T 'T ' T
'
r-..; . BOlhariya Formation
Radwan Formation (Oligocene)
Green sandstone
Bahariya Formation (Lowar Cenom~nian)
Fig, 2. Columnar sections show the occurrences of green sand stone at G. Radwan and Limestone Hill.
Origin and occurrence of glauconite in the green sandstone associated with unconformity
Fig. 3. Glauconite patches (dark) extensively replaces the massive calcite (clear). Notice the sharp contact between glauconite and quartz. G. Radwan, O.L. x-63. Fig. -to Glauconite progressively replaces the massive calcite along its cleavage giving gluconite streaks. G. Radwan, O.L. x-63.
323
M. A.
KHALIFA
6
Fig. 5. Glauconite coatings (dark) resulted from preferential repl acement of silica overgrowth around quartz. Limestone Hill (lower bed), O .L. x-es, Fig. 6. Unzoned diagenetic calcite replaced both glauconite coating and qu artz (arrows). Notice the glauconite coating in the lower part of the figure. Limestone Hill (lower bed), O.L. x -{)3. Fig. 7. Glauconite patches repl aced quartz grains. Notice their replacement with calcite (clear). Limestone Hill (uppcrbed), O.L. x-{)3.
324
Origin and occurrence of glauconite in the green sandstone associated with unconformity other th an quartz, and vice versa. These conditions may give a more or less clear picture about the environment that prevailed during the glauconitization of calcite and quartz. At Gabel Radwan glauconite was selectively replacing the early deposited massive and blocky calcite cement. This preference may be directly related to the solubility of calcite which is more soluble than quartz under such conditions. The instability of calcite is a function of physico-chemical factors such as pressure, temperature and pH values. Low temperature may result in the dissolution of calcite, this stems from the fact that cold water holds more calcium carbonate than warm water (Sharma 1965). Changes in the pH value of fluids of the sediments in nature show a reversible relationship with the amount of carbon dioxide dissolved in them. Consequently, the presence of high amounts of carbon dioxide minimizes the pH values and keeps calcium carbonate dissolved in solution (Sharma 1965). The above physico-chemical factors (low temperature and pH) most probably prevail ncar the exposed surface, this is in harmony with the conclusion of Graf and Lamar (1950) and Adams (1964) who observed that calcite cem ent in outcrop is unstable and dissolved by weathering . Hence, the replacement of calcite by glauconite in the unconformity surface may take place under subaerial conditions. The complete and near complete replacement of calcite by glauco nite suggests that glauconite prefers to be formed in the unconformity surface in a subaerial environment, where , available conditions may be prevailed or dominated. The available condition may imply the presence of appreciable amounts of K, AI and Fe which a re needed for the formation of glauconite. AI Gailani (1980) made an identification of unconformity by using semi-quantitative chemical analysis . He found that there was an abrupt increase in K and AI in the unconformity. The present author believes that the abrupt increase in K, AI and Fe in the unconformity at G. Radwan may be derived from the weathering and disintegration of clays and feldspars of the underlying Bahariya Formation. Thi s opinion confirms the conclusion of Takahashi and Vagi (1929) and Takahashi (1939), who proposed that the parent material for glauconitization was the silicates and aluminates derived by the hydration of various silicates. In contrast, at Limestone Hill , the glauconite prefers to replace the syntaxial silica overgrowths in the lower bed and quartz grains in the upper one . Hence, the physico-chemical factors in the present case are quite different (reversible) to the above example at G. Radwan. In that case, quartz is unstable and is more soluble than calcite. The relative instability of quartz in the presence of calcite may be attributed to the increase in temperature (Dapples 1959, Siever 1959). It was found that the relative increase in temperature of the sediments may result from burial of these sediments. In the latter case, the supply of carbon dioxide decreases A!S 1:J/ 4- J
325
and the pH rises, resulting in the dissolution of silica and precipitation of calcite (Sh arma 1965). In the light of the above physico-chemical conditions that have been dominated in the buried sediments, the preferential glauconitization of silica at the Limestone Hill only, may proceed under subcrop conditions away from the weathered zone. In such a case, the incipient glauconitization of silica may take place in the very late phase of diagenesis afte i ,lithification and burial. The partial glauconitization of silica in the subcrop condition may indicate a depletion and limit ed supply of K, AI and Fe.
CONCLUSION In the present study, (1) glauconite can be formed diagcnetically in the form of coatings, streaks and . patches on the continental environments in subaerial and subcrop conditions. (2) It prefers to replace the massive calcite cement in subaerial conditions (in unconformity surface) under low temperature, pressure 'a nd pH . (3) On the other hand ,glauconite selectively replaces the silica overgrowths and some quartz grains in subcrop conditions (buried sediments) under high pressure, temperature and pH values. (4) Extensive glauconitization of calcite in the unconformity surface may be attributed to the plentiful supply of K , AI and Fe released from the weathered and disintegrated clays and . feldspars in subaerial conditions. (5) The partial glauconitization in subcrop conditions may result from the depletion of K, AI and Fe. (6) Glauconite can be used as a clue for the recognition of unconformities.
REFERENCES Ad ams, W. L. 1964. Diagenetic aspects of Lower Morrowan, Pennsylvanian sandstones, Northwestern Okl ahoma. Bull. AII/. Ass. Petrol Geol. 48, 1568-1580. Al-Gail ani , M. B. 1980. Geochem ical ident ification of unconformities using semi-quantitative X-ray fluor escence analysis. J. Sedim . Petrol. 50. 1261-1270. Dapples, E. C. 1959. The behavi or of silica in diagenesis. In: Silica ill Sedim ents (edited by Ireland , H. A .). Soc. Econ . Palacontologist and Mineral ogists Spec . Publ. 7,36-54. Goldman. M. I. 1921. Asso ciation of glauconite with unconformities. Geol. Soc. Am. 32.25. Gr af, D . L. and Lamar, J. E. 1950. Petrology of Fredomia oolit e in Southern Illinois . BIiI/. AII/. A ss. Petrol. Geol. 34, 2318-233 6. Mcsolclla , K. J . 1965. Collophane associated with the unco nform ity at the base of the Devonian Onondaga Limestone in New York State. J . Sedim. Petrol. 35,260-262. Said. R. t962. The Geology of Egypt . Elsevier, Am sterdam. Said, R . and Issawi, B. 1964, Geol ogy of north ern plateau, Bahariy a Oase s , Egypt. GeDI. Sun'. Egypt , pap er No. 29. Sharma, G. D. 1965. Form at ion of silica cement and its repla cem ent by carbonates. J. Sedim . Petrol. 35,733-745. Siever , R. 1959. Petrology and geochem istry of silica cementation in some Penns ylvania sand stone . In: Silica ill Sediment (edited by Ireland, H. A .), Soc. Econ . Paleon tologists and t-.lineralogists Spec. Publ. 7,55-79. Takahashi , J . I. 1939. Synopsis of glauconitization in Recent marine sediments. Bull. Am. Ass. Petrol. Gcol. 23. Takahashi. J. I. and Yagi, 1929. Pccular mud-grains and their relation to the origin of glauconite. Econ . Gcol. 24. 838-852.
Journal ofAfrican Earth Sciences. Vol. I. No. 3/-t. pp. 321 t0325. 1983 Printed in Great Britain
Origin and occurrence of glauconite in the green sandstone associated with unconformity, Bahariya Oases, Western Desert, Egypt M. A.
KHALIFA
Menoufia University, Shibin El Kom, Egypt
(Received 13 October 1983) Abstract-This paper presents a new concept concerning the formation of authigenic glauconite in the unconformity surfaces. It is usually formed diagcnetically in subaerial and subcrop conditions on continental environments. In subaerial conditions glauconite selectively only replaces the massive and blocky calcite cements that formed in early stages of diagenesis. This may be attributed to the increase of the solubility of calcite at, and ncar, the weathered zone, under low temperature, pressure and pH values. On the other hand, in subcrop conditions glauconite prefers to replace the silica overgrowth around quartz grain without any replacement of the diagnetie zoned calcite cement. In such case, a reversible condition can be expected such as the increase of the solubility of silica under high temperature, pressure and pH values.
INTRODUCTION
OCCURRENCE OF GLAUCONITE
LITILE is known about the origin and occurrence of glauconite in the unconformity surface, except for the work of Goldman (1921) who recognized association of glauconite in the unconformity surface. Mesolella (1965), also found authigenic glauconite replacing nodules of collophane in the unconformity surface at the base of the Devonian Onondaga Limestone in New York State. The green sandstones are often encountered at and near the major unconformity that separates the Middle Cretaceous from Tertiary sediments in the Bahariya Oases. This is clearly in evidence at Gabel Radwan and Limestone Hill (Fig. 1). At Gabel Radwan (Fig. 2), the green sandstone separates the Lower Cenomanian Bahariya Formation (Said 1962) from the Oligocene Radwan Formation (Said and Issawi 1964). At this locality, the green sandstone has a deep green to bluish green, and is coarse (0.6 mm) to very coarse (1.2 mm) grained, poorly sorted and very hard. It attains a thickness of about one meter and extends laterally for several kilometers. At limestone Hill, the green sandstones are represented by two thin beds, each of which has an average thickness of about 45 cm. These beds rest unconformably on the Lower Cenomanian Bahariya Formation and conformably underlie the calcareous sandstones and limestones (Post Eocene?) that form the cape rock of the Limestone Hill (Fig. 2). The intention of this paper is to explain the causes of the restricted occurrence of glauconite in the unconformity surface. It calls attention to the possible generation of glauconite in the continental environments instead of marine conditions. Also, it possibly points out a new direction of work which could prove helpful in solving the problem of the physico-chemical condition needed for the glauconite formation.
Petrographic investigation of representative samples from Gabel Radwan show that. glauconite occurs as coatings, streaks and patches. Most of these glauconites have been produced entirely from the replacement of early deposited massive and blocky calcite cement. At the same time, glauconite does not replace quartz grains, where a sharp contact between them can be noticed (Fig. 3). Glauconite coatings may proceed at the contact between the calcite and quartz grains. In a further glauconitization process, glauconite encroaches and invades through the massive calcite along their cleavage giving the glauconite streaks (Fig. 4). Complete replacement of massive calcite by glauconite may produce glauconite patches (Fig. 3). At Limestone Hill, especially in the lower bed, glauconite selectively replaces the syntaxial silica overgrowth around quartz grains (Fig. 5), forming a glauconite coating. At the same time, glauconite does not show any textural replacement of the diagenetic zoned massive calcite cement, but it has been remarked that a new generation of unzoned calcite replaces the glauconite and even the quartz grains (Fig. 6). In the upper bed, glauconite commonly replaces the quartz grains in the form of irregular patches and coatings (Fig. 7). Most of these patches have been replaced by microspar and spray calcite; this is evidenced by the invasion of clear calcite patches within the dark green glauconite (Fig. 7).
ORIGIN AND DISCUSSION The most important factors used for the explanation of glauconite genesis in the present study, are the physico-chemical conditions (temperature, pressure and pH) under which glauconite prefers to replace calcite
321
M, A.
322
,
1f
KHALIFA
1,
o Ain EI Hiz
2d-__--J~---__#C-----_t_-----128
•
29
Fig. I, Location map of the studied area,
LIMESTONE HILL
G,RAOWAN T OT . T
'
r 'T , "T '
T 'T . T ' T 'T
' T ' T 'T
Green sandstones
'T '
T 'T ' T
'
r-..; . BOlhariya Formation
Radwan Formation (Oligocene)
Green sandstone
Bahariya Formation (Lowar Cenom~nian)
Fig, 2. Columnar sections show the occurrences of green sand stone at G. Radwan and Limestone Hill.
Origin and occurrence of glauconite in the green sandstone associated with unconformity
Fig. 3. Glauconite patches (dark) extensively replaces the massive calcite (clear). Notice the sharp contact between glauconite and quartz. G. Radwan, O.L. x-63. Fig. -to Glauconite progressively replaces the massive calcite along its cleavage giving gluconite streaks. G. Radwan, O.L. x-63.
323
M. A.
KHALIFA
6
Fig. 5. Glauconite coatings (dark) resulted from preferential repl acement of silica overgrowth around quartz. Limestone Hill (lower bed), O .L. x-es, Fig. 6. Unzoned diagenetic calcite replaced both glauconite coating and qu artz (arrows). Notice the glauconite coating in the lower part of the figure. Limestone Hill (lower bed), O.L. x -{)3. Fig. 7. Glauconite patches repl aced quartz grains. Notice their replacement with calcite (clear). Limestone Hill (uppcrbed), O.L. x-{)3.
324
Origin and occurrence of glauconite in the green sandstone associated with unconformity other th an quartz, and vice versa. These conditions may give a more or less clear picture about the environment that prevailed during the glauconitization of calcite and quartz. At Gabel Radwan glauconite was selectively replacing the early deposited massive and blocky calcite cement. This preference may be directly related to the solubility of calcite which is more soluble than quartz under such conditions. The instability of calcite is a function of physico-chemical factors such as pressure, temperature and pH values. Low temperature may result in the dissolution of calcite, this stems from the fact that cold water holds more calcium carbonate than warm water (Sharma 1965). Changes in the pH value of fluids of the sediments in nature show a reversible relationship with the amount of carbon dioxide dissolved in them. Consequently, the presence of high amounts of carbon dioxide minimizes the pH values and keeps calcium carbonate dissolved in solution (Sharma 1965). The above physico-chemical factors (low temperature and pH) most probably prevail ncar the exposed surface, this is in harmony with the conclusion of Graf and Lamar (1950) and Adams (1964) who observed that calcite cem ent in outcrop is unstable and dissolved by weathering . Hence, the replacement of calcite by glauconite in the unconformity surface may take place under subaerial conditions. The complete and near complete replacement of calcite by glauco nite suggests that glauconite prefers to be formed in the unconformity surface in a subaerial environment, where , available conditions may be prevailed or dominated. The available condition may imply the presence of appreciable amounts of K, AI and Fe which a re needed for the formation of glauconite. AI Gailani (1980) made an identification of unconformity by using semi-quantitative chemical analysis . He found that there was an abrupt increase in K and AI in the unconformity. The present author believes that the abrupt increase in K, AI and Fe in the unconformity at G. Radwan may be derived from the weathering and disintegration of clays and feldspars of the underlying Bahariya Formation. Thi s opinion confirms the conclusion of Takahashi and Vagi (1929) and Takahashi (1939), who proposed that the parent material for glauconitization was the silicates and aluminates derived by the hydration of various silicates. In contrast, at Limestone Hill , the glauconite prefers to replace the syntaxial silica overgrowths in the lower bed and quartz grains in the upper one . Hence, the physico-chemical factors in the present case are quite different (reversible) to the above example at G. Radwan. In that case, quartz is unstable and is more soluble than calcite. The relative instability of quartz in the presence of calcite may be attributed to the increase in temperature (Dapples 1959, Siever 1959). It was found that the relative increase in temperature of the sediments may result from burial of these sediments. In the latter case, the supply of carbon dioxide decreases A!S 1:J/ 4- J
325
and the pH rises, resulting in the dissolution of silica and precipitation of calcite (Sh arma 1965). In the light of the above physico-chemical conditions that have been dominated in the buried sediments, the preferential glauconitization of silica at the Limestone Hill only, may proceed under subcrop conditions away from the weathered zone. In such a case, the incipient glauconitization of silica may take place in the very late phase of diagenesis afte i ,lithification and burial. The partial glauconitization of silica in the subcrop condition may indicate a depletion and limit ed supply of K, AI and Fe.
CONCLUSION In the present study, (1) glauconite can be formed diagcnetically in the form of coatings, streaks and . patches on the continental environments in subaerial and subcrop conditions. (2) It prefers to replace the massive calcite cement in subaerial conditions (in unconformity surface) under low temperature, pressure 'a nd pH . (3) On the other hand ,glauconite selectively replaces the silica overgrowths and some quartz grains in subcrop conditions (buried sediments) under high pressure, temperature and pH values. (4) Extensive glauconitization of calcite in the unconformity surface may be attributed to the plentiful supply of K , AI and Fe released from the weathered and disintegrated clays and . feldspars in subaerial conditions. (5) The partial glauconitization in subcrop conditions may result from the depletion of K, AI and Fe. (6) Glauconite can be used as a clue for the recognition of unconformities.
REFERENCES Ad ams, W. L. 1964. Diagenetic aspects of Lower Morrowan, Pennsylvanian sandstones, Northwestern Okl ahoma. Bull. AII/. Ass. Petrol Geol. 48, 1568-1580. Al-Gail ani , M. B. 1980. Geochem ical ident ification of unconformities using semi-quantitative X-ray fluor escence analysis. J. Sedim . Petrol. 50. 1261-1270. Dapples, E. C. 1959. The behavi or of silica in diagenesis. In: Silica ill Sedim ents (edited by Ireland , H. A .). Soc. Econ . Palacontologist and Mineral ogists Spec . Publ. 7,36-54. Goldman. M. I. 1921. Asso ciation of glauconite with unconformities. Geol. Soc. Am. 32.25. Gr af, D . L. and Lamar, J. E. 1950. Petrology of Fredomia oolit e in Southern Illinois . BIiI/. AII/. A ss. Petrol. Geol. 34, 2318-233 6. Mcsolclla , K. J . 1965. Collophane associated with the unco nform ity at the base of the Devonian Onondaga Limestone in New York State. J . Sedim. Petrol. 35,260-262. Said. R. t962. The Geology of Egypt . Elsevier, Am sterdam. Said, R . and Issawi, B. 1964, Geol ogy of north ern plateau, Bahariy a Oase s , Egypt. GeDI. Sun'. Egypt , pap er No. 29. Sharma, G. D. 1965. Form at ion of silica cement and its repla cem ent by carbonates. J. Sedim . Petrol. 35,733-745. Siever , R. 1959. Petrology and geochem istry of silica cementation in some Penns ylvania sand stone . In: Silica ill Sediment (edited by Ireland, H. A .), Soc. Econ . Paleon tologists and t-.lineralogists Spec. Publ. 7,55-79. Takahashi , J . I. 1939. Synopsis of glauconitization in Recent marine sediments. Bull. Am. Ass. Petrol. Gcol. 23. Takahashi. J. I. and Yagi, 1929. Pccular mud-grains and their relation to the origin of glauconite. Econ . Gcol. 24. 838-852.