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Chemical weathering and the formation of pseudo-karst topography in the Roraima Group, Gran Sabana, Venezuela
Chemical Geology, 107 (1993) 341-343 Elsevier Science Publishers B.V., Amsterdam
341
Chemical weathering and the formation of pseudo-karst topography in the Roraima Group, Gran Sabana, Venezuela C.E. Yanes and H.O. Bricefio lnstituto de Ciencias de la Tierra, Facultad de Ciencias, Caracas, Universidad Central de Venezuela, Apartado 3895, Caracas 1010-A, Venezuela
(Received March 19, 1993; revised and accepted March 30, 1993)
The Gran Sabana region, located in the southeastern part of the Venezuelan Guayana Shield, is characterized by several spectacular table-mountains, whose elevations above sea level range from 1800 to > 2600 m, and by savannah- or jungle-covered lowlands (Schubert and Huber, 1989). These table-mountains are remnants of the Auy~in-tepuy planation surface, with a speculative Mesozoic age of origin. The surrounding lowlands are located on the Wonken planation surface (Bricefio and Schubert, 1990). The morphology of the tablemountain summits conforms to the definition of a karst morphology. It is characterized by chaotic block accumulations, walls, arches and towers, tower field, depressions, caves, and residual forms (Bricefio and Schubert, 1990). These forms have been developed mainly; on the low-grade metasedimentary rocks of the Precambrian Roraima Group (1.6 Ga) that were intruded by diabase dikes and sills (Gonz~ilez de Juana et al., 1980; Gibbs and Barron, 1983). Geologically, the rocks of the Roraima Group consist of meta-sandstones (arkoses, feldspathic sandstones, volcaniclastic sandstones, quartzitic sandstones, quartzitic wackes, subarkoses, lithic wackes and wackes ) with abundant crossbedding; polymictic metaconglomerates, meta-siltstones and siliceous rocks (chert and jasper) (Gibbs and Barron, 1983).
The Upper Caroni Basin includes extensive areas of the Gran Sabana region. The Carrao, Aponguao, Yuruani, Caruai and Kukenan rivers are the principal tributaries. Rainfall in the Gran Sabana region ranges from 2000 to > 3000 m m yr-~, and the rainy and dry seasons are not well defined. The annual mean temperature is 23 °C. In the region, the resistant sandstones have been affected by karstification through geologic time (probably up to ~ 70 Ma; Bricefio et al., 1990). It has been proposed (Martini, 1982; Urbani, 1986; Bricefio et al., 1990) that chemical weathering, together with the constant removal of detritus has produced a karstlike topography on the table-mountain summits. Very little geochemical research has been done in the Gran Sabana region, except for those investigations of Grupo Cientifico Chimant~i ( 1986 ) and Bricefio et al. ( 1988, 1990 ). Recently, a water and suspended-load sampiing campaign was performed in the Upper Caroni Basin; to establish the annual rates of chemical weathering for sedimentary rocks under tropical conditions. This information will be very useful in estimating the chemical weathering rate for the whole basin and hence achieving a better understanding of the hydrogeochemical processes that produced the spectacular table-mountain and savannah land-
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
342
scapes of the Gran Sabana. The results of this campaign are the subject of the present report. The Caroni River, including its main tributaries and minor streams located in the Upper Caroni Basin, have been monitored for water quality and for particulate matter composition since 1985. All sampling sites were located near the mouth of each river, except for the Caroni River which was sampled in Caruaiken. This site represents the load from the Upper Caroni Basin. Integrated water samples were collected at each river. At the end of each field day, samples were filtered with 0.45-/zm filters aid nitric acid was added to preserve them. The samples were then air-tightened and frozen. Rainwater samples were collected biweekly in two localities, San Ignacio de Yuruani and Wonken. The collector device had a surface area of 490 c m 2 and was mounted on a tower, 3 m above the ground. Standard tests were performed to establish physico-chemical parameters (pH, temperature and conductivity). Suspended sediment samples were obtained by using a portable continuous flow centrifuge and a ultrafiltration system. Chemical analyses of water samples (rain and river) were performed by atomic absorption spectrometry (AAS) (Ca, Na, K, Mg, A1, Fe, Mn), colorimetry (Si), and ion chromatography (SO42-, CI-, HCO3-, NO~-). The amounts of DOC (dissolved organic carbon) in river water was determined by combustionchromatography. Chemical and mineralogical analyses of suspended materials were performed by energy-dispersive X-ray analyses (EDAX), AAS and X-ray diffraction ( X R D ) . Rainwaters collected in the Gran Sabana (San Ignacio de Yuruani and Wonken) have very homogeneous compositions (i.e. Na ÷, Mg 2+, Ca 2+, C1- ) and pH. This suggests that rain sources are relatively well distributed. The volume weighted average (VWA) composition of rainwater in Gran Sabana can be described as a slightly acidic (pH 4.60). Major ions in these waters are of the same order as
C.E YANES AND H.O BRICEIqO
concentrations observed at other sites in tropical America. In this study, the annual average concentration for each of the dissolved components and the suspended solids, estimated for each stream in are flow-weighted, and the mean chemical composition of suspended sediments are massweighted. The lithological contribution was calculated by subtracting the rainwater and atmospheric inputs from the tetal dissolved solids (TDS). In this manner, the chemical weathering rate was estimated for each river, integrated for the Carrao Basin and the Upper Caroni Basin and, finally, for the Gran Sabana region as whole. Rivers draining the Gran Sabana region are acidic (pH as low as 4.9) due to the presence of organic acids derived from the jungle environment. They are characterized by high concentrations of DOC and silica (SIO2) and have very low TDS. These characteristics are indicative of a region where rocks were deeply weathered under transport-limited weathering (Stallard and Edmond, 1987), and intensive organic matter decomposition is taking place. The major dissolved ions of these rivers are Na ÷, K +, HCO~-, Ca 2+, C1- and Mg 2÷. The higher concentrations of calcium and magnesium were detected in those rivers draining diabase terrane. Sodium and potassium are supplied by the process of decomposition of feldspar grains of the sandstones, and from rainwater. Chloride and sulphate come, almost exclusively, from rainwater. Chemical weathering rates values obtained for the Carrao Basin, the Upper Caroni Basin and Gran Sabana region were 6.2, 6.9 and 6.6 t km -2 yr-1, respectively. These small differences found in weathering rates are due mainly to the similar runoff distribution and lithological characteristics observed for the basins. The S i / ( N a + K ) ratios for rivers draining the Gran Sabana region provide a good measure of the intensity of weathering within each drainage basin. These ratios range from 1.8 to 3.7. This indicates that weathering of a mate-
PSEUDO-KARST TOPOGRAPHY IN THE RORAIMA GROUP, GRAN SABANA, VENEZUELA
rial with normative mineralogy equivalent to gibbsite and quartz, or weathering of primary aluminium silicates to kaolinite a n d / o r gibbsite seem to be the relevant transformation in the basins, highlighting a very intense weathering environment. This is in agreement with: ( 1 ) the low weathering rate estimated for the Gran Sabana region; (2) the abundance of kaolinite, quartz and lesser quantities of gibbsite in the suspended sediments and soil profiles; and (3) the chemical composition of the suspended matter (SiO2, A1203, Fe203 as high as 72% and Zr as the most abundant trace element ). Chemical weathering of the siliceous cement, followed by mechanical removal of detritus has been proposed as the mechanism to explain the development of karst-like structures in the Roraima meta-sandstones. According to calculations by Bricefio et al. (1990), rock weathering, through a process of solution of the siliceous cement ( ~ 20% of the total rock), produces a denudation of 2800 m in 70 Ma, under the present climatic conditions. This estimation assumes that the process is 100% efficient, and that the concentration of silica in the water reaches 6 mg lHowever, the chemical composition of river waters draining the Gran Sabana region indicates that concentration of silica by dissolution of quartz reaches barely 2.0 mg 1- ~, so silica saturation is not reached. This means that to achieve the denudation rates proposed by Bricefio et al. (1990), we must consider the weathering of other components in the system, such as feldspars to produce kaolinite. In other words, weathering probably took place dissolving silica cement and feldspar grains together. The actual denudation rate, derived from our data in the Gran Sabana region, is on the order of 34 t k m -2 y r - ~. U n d e r current cli-
343
matic conditions, these denudation rates would imply a topographic lowering of 907 m in a 70Ma period. We believe that the development of karst features can be explained by chemical weathering of the siliceous cement and feldspar grains, followed by mechanical removal of sand grains under vadose conditions. The result of this process is a progressive arenisation of the rock along zones of weakness such as joints and bedding planes (Urbani, 1986). References Bricefio, H.O. and Schubert, C., 1990. Geomorphology of the Gran Sabana Guayana Shield, southeastern Venezuela. Geomorphology, 3:125-141. Bricefio, H.O., Yanes, C.E., Ramirez, A., Martinez, E., Lopez, C. and Tirado, O., 1988. Geochemistry of the Caroni-Paragua Basin. In: E.T. Degens, S. Kempe and S. Naiden (Editors), Transport of Carbon and Minerals in Major World Rivers. Mitt. Geol. Pal~iontol. Inst. Univ. Hamburg, SCOPE/UNEP Sonderband, 66: 415-422. Bricefio, H.O., Schubert, C. and Paolini, J., 1990. Tablemountain geology and surficial geochemistry: Chimanta Massif, Venezuelan Guayana Shield. J. S. Am. Earth Sci., 3: 179-194. Gibbs, A.K. and Barron, C.N., 1983. The Guiana Shield reviewed. Episodes, 2: 7-14. Gonzalez de Juana, C., lturralde de Arozena, J.M. and Picard, X., 1980. Geologia de Venezuela y de sus cuencas petroliferas. FONINVES, Caracas, 1031 pp. Grupo Cientifico Chimant~, 1986. Reconocimiento prcliminar del Macizo de Chimanta, Estado Bolivar. Venezuela. Acta Cient. Venez., 37: 25-42. Martini, J.E., 1982. Karst in Black Reef and Wolkenberg Group quarzite of eastern Transvaal escarpment. Bol. Soc. Venez. Espeleol., 10:99-114. Schubert, C. and Huber, O., 1989. La Gran Sabana: Panoramica de vua regi6n. Cuadernos Lagoven, Caracas, 207 pp. Stallard, R.F. and Edmond, J.M., 1987. Geochemistry of the Amazon, 3. Weathering chemistry and limits to dissolved inputs. J. Geophys. Res., 92: 8293-8302. Urbani, F., 1986. Notas sobre el origen de las cavidades en rocas cuarclferas prec~imbricas del Grupo Roraima. Interciencia, 11 : 298-300.
341
Chemical weathering and the formation of pseudo-karst topography in the Roraima Group, Gran Sabana, Venezuela C.E. Yanes and H.O. Bricefio lnstituto de Ciencias de la Tierra, Facultad de Ciencias, Caracas, Universidad Central de Venezuela, Apartado 3895, Caracas 1010-A, Venezuela
(Received March 19, 1993; revised and accepted March 30, 1993)
The Gran Sabana region, located in the southeastern part of the Venezuelan Guayana Shield, is characterized by several spectacular table-mountains, whose elevations above sea level range from 1800 to > 2600 m, and by savannah- or jungle-covered lowlands (Schubert and Huber, 1989). These table-mountains are remnants of the Auy~in-tepuy planation surface, with a speculative Mesozoic age of origin. The surrounding lowlands are located on the Wonken planation surface (Bricefio and Schubert, 1990). The morphology of the tablemountain summits conforms to the definition of a karst morphology. It is characterized by chaotic block accumulations, walls, arches and towers, tower field, depressions, caves, and residual forms (Bricefio and Schubert, 1990). These forms have been developed mainly; on the low-grade metasedimentary rocks of the Precambrian Roraima Group (1.6 Ga) that were intruded by diabase dikes and sills (Gonz~ilez de Juana et al., 1980; Gibbs and Barron, 1983). Geologically, the rocks of the Roraima Group consist of meta-sandstones (arkoses, feldspathic sandstones, volcaniclastic sandstones, quartzitic sandstones, quartzitic wackes, subarkoses, lithic wackes and wackes ) with abundant crossbedding; polymictic metaconglomerates, meta-siltstones and siliceous rocks (chert and jasper) (Gibbs and Barron, 1983).
The Upper Caroni Basin includes extensive areas of the Gran Sabana region. The Carrao, Aponguao, Yuruani, Caruai and Kukenan rivers are the principal tributaries. Rainfall in the Gran Sabana region ranges from 2000 to > 3000 m m yr-~, and the rainy and dry seasons are not well defined. The annual mean temperature is 23 °C. In the region, the resistant sandstones have been affected by karstification through geologic time (probably up to ~ 70 Ma; Bricefio et al., 1990). It has been proposed (Martini, 1982; Urbani, 1986; Bricefio et al., 1990) that chemical weathering, together with the constant removal of detritus has produced a karstlike topography on the table-mountain summits. Very little geochemical research has been done in the Gran Sabana region, except for those investigations of Grupo Cientifico Chimant~i ( 1986 ) and Bricefio et al. ( 1988, 1990 ). Recently, a water and suspended-load sampiing campaign was performed in the Upper Caroni Basin; to establish the annual rates of chemical weathering for sedimentary rocks under tropical conditions. This information will be very useful in estimating the chemical weathering rate for the whole basin and hence achieving a better understanding of the hydrogeochemical processes that produced the spectacular table-mountain and savannah land-
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
342
scapes of the Gran Sabana. The results of this campaign are the subject of the present report. The Caroni River, including its main tributaries and minor streams located in the Upper Caroni Basin, have been monitored for water quality and for particulate matter composition since 1985. All sampling sites were located near the mouth of each river, except for the Caroni River which was sampled in Caruaiken. This site represents the load from the Upper Caroni Basin. Integrated water samples were collected at each river. At the end of each field day, samples were filtered with 0.45-/zm filters aid nitric acid was added to preserve them. The samples were then air-tightened and frozen. Rainwater samples were collected biweekly in two localities, San Ignacio de Yuruani and Wonken. The collector device had a surface area of 490 c m 2 and was mounted on a tower, 3 m above the ground. Standard tests were performed to establish physico-chemical parameters (pH, temperature and conductivity). Suspended sediment samples were obtained by using a portable continuous flow centrifuge and a ultrafiltration system. Chemical analyses of water samples (rain and river) were performed by atomic absorption spectrometry (AAS) (Ca, Na, K, Mg, A1, Fe, Mn), colorimetry (Si), and ion chromatography (SO42-, CI-, HCO3-, NO~-). The amounts of DOC (dissolved organic carbon) in river water was determined by combustionchromatography. Chemical and mineralogical analyses of suspended materials were performed by energy-dispersive X-ray analyses (EDAX), AAS and X-ray diffraction ( X R D ) . Rainwaters collected in the Gran Sabana (San Ignacio de Yuruani and Wonken) have very homogeneous compositions (i.e. Na ÷, Mg 2+, Ca 2+, C1- ) and pH. This suggests that rain sources are relatively well distributed. The volume weighted average (VWA) composition of rainwater in Gran Sabana can be described as a slightly acidic (pH 4.60). Major ions in these waters are of the same order as
C.E YANES AND H.O BRICEIqO
concentrations observed at other sites in tropical America. In this study, the annual average concentration for each of the dissolved components and the suspended solids, estimated for each stream in are flow-weighted, and the mean chemical composition of suspended sediments are massweighted. The lithological contribution was calculated by subtracting the rainwater and atmospheric inputs from the tetal dissolved solids (TDS). In this manner, the chemical weathering rate was estimated for each river, integrated for the Carrao Basin and the Upper Caroni Basin and, finally, for the Gran Sabana region as whole. Rivers draining the Gran Sabana region are acidic (pH as low as 4.9) due to the presence of organic acids derived from the jungle environment. They are characterized by high concentrations of DOC and silica (SIO2) and have very low TDS. These characteristics are indicative of a region where rocks were deeply weathered under transport-limited weathering (Stallard and Edmond, 1987), and intensive organic matter decomposition is taking place. The major dissolved ions of these rivers are Na ÷, K +, HCO~-, Ca 2+, C1- and Mg 2÷. The higher concentrations of calcium and magnesium were detected in those rivers draining diabase terrane. Sodium and potassium are supplied by the process of decomposition of feldspar grains of the sandstones, and from rainwater. Chloride and sulphate come, almost exclusively, from rainwater. Chemical weathering rates values obtained for the Carrao Basin, the Upper Caroni Basin and Gran Sabana region were 6.2, 6.9 and 6.6 t km -2 yr-1, respectively. These small differences found in weathering rates are due mainly to the similar runoff distribution and lithological characteristics observed for the basins. The S i / ( N a + K ) ratios for rivers draining the Gran Sabana region provide a good measure of the intensity of weathering within each drainage basin. These ratios range from 1.8 to 3.7. This indicates that weathering of a mate-
PSEUDO-KARST TOPOGRAPHY IN THE RORAIMA GROUP, GRAN SABANA, VENEZUELA
rial with normative mineralogy equivalent to gibbsite and quartz, or weathering of primary aluminium silicates to kaolinite a n d / o r gibbsite seem to be the relevant transformation in the basins, highlighting a very intense weathering environment. This is in agreement with: ( 1 ) the low weathering rate estimated for the Gran Sabana region; (2) the abundance of kaolinite, quartz and lesser quantities of gibbsite in the suspended sediments and soil profiles; and (3) the chemical composition of the suspended matter (SiO2, A1203, Fe203 as high as 72% and Zr as the most abundant trace element ). Chemical weathering of the siliceous cement, followed by mechanical removal of detritus has been proposed as the mechanism to explain the development of karst-like structures in the Roraima meta-sandstones. According to calculations by Bricefio et al. (1990), rock weathering, through a process of solution of the siliceous cement ( ~ 20% of the total rock), produces a denudation of 2800 m in 70 Ma, under the present climatic conditions. This estimation assumes that the process is 100% efficient, and that the concentration of silica in the water reaches 6 mg lHowever, the chemical composition of river waters draining the Gran Sabana region indicates that concentration of silica by dissolution of quartz reaches barely 2.0 mg 1- ~, so silica saturation is not reached. This means that to achieve the denudation rates proposed by Bricefio et al. (1990), we must consider the weathering of other components in the system, such as feldspars to produce kaolinite. In other words, weathering probably took place dissolving silica cement and feldspar grains together. The actual denudation rate, derived from our data in the Gran Sabana region, is on the order of 34 t k m -2 y r - ~. U n d e r current cli-
343
matic conditions, these denudation rates would imply a topographic lowering of 907 m in a 70Ma period. We believe that the development of karst features can be explained by chemical weathering of the siliceous cement and feldspar grains, followed by mechanical removal of sand grains under vadose conditions. The result of this process is a progressive arenisation of the rock along zones of weakness such as joints and bedding planes (Urbani, 1986). References Bricefio, H.O. and Schubert, C., 1990. Geomorphology of the Gran Sabana Guayana Shield, southeastern Venezuela. Geomorphology, 3:125-141. Bricefio, H.O., Yanes, C.E., Ramirez, A., Martinez, E., Lopez, C. and Tirado, O., 1988. Geochemistry of the Caroni-Paragua Basin. In: E.T. Degens, S. Kempe and S. Naiden (Editors), Transport of Carbon and Minerals in Major World Rivers. Mitt. Geol. Pal~iontol. Inst. Univ. Hamburg, SCOPE/UNEP Sonderband, 66: 415-422. Bricefio, H.O., Schubert, C. and Paolini, J., 1990. Tablemountain geology and surficial geochemistry: Chimanta Massif, Venezuelan Guayana Shield. J. S. Am. Earth Sci., 3: 179-194. Gibbs, A.K. and Barron, C.N., 1983. The Guiana Shield reviewed. Episodes, 2: 7-14. Gonzalez de Juana, C., lturralde de Arozena, J.M. and Picard, X., 1980. Geologia de Venezuela y de sus cuencas petroliferas. FONINVES, Caracas, 1031 pp. Grupo Cientifico Chimant~, 1986. Reconocimiento prcliminar del Macizo de Chimanta, Estado Bolivar. Venezuela. Acta Cient. Venez., 37: 25-42. Martini, J.E., 1982. Karst in Black Reef and Wolkenberg Group quarzite of eastern Transvaal escarpment. Bol. Soc. Venez. Espeleol., 10:99-114. Schubert, C. and Huber, O., 1989. La Gran Sabana: Panoramica de vua regi6n. Cuadernos Lagoven, Caracas, 207 pp. Stallard, R.F. and Edmond, J.M., 1987. Geochemistry of the Amazon, 3. Weathering chemistry and limits to dissolved inputs. J. Geophys. Res., 92: 8293-8302. Urbani, F., 1986. Notas sobre el origen de las cavidades en rocas cuarclferas prec~imbricas del Grupo Roraima. Interciencia, 11 : 298-300.