Scielo RSS <![CDATA[Latin American journal of sedimentology and basin analysis]]> http://www.scielo.org.ar/rss.php?pid=1851-497920110001&lang=en vol. 18 num. 1 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <link>http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000100001&lng=en&nrm=iso&tlng=en</link> <description/> </item> <item> <title><![CDATA[Petrología y evolución diagenética de las facies silicoclásticas del Grupo Sierras Bayas, Sistema de Tandilia, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000100002&lng=en&nrm=iso&tlng=en El estudio de las características petrológicas y microfaciales de las unidades silicoclásticas marinas del Neoproterozoico del Grupo Sierras Bayas, (en orden estratigráfico ascendente: formaciones Villa Mónica, Cerro Largo y Olavarría), se presenta en este trabajo con la finalidad de evaluar la evolución e importancia de los controles producidos durante los regímenes eodiagenético, mesodiagenético y telodiagenético. En la sección basal de la Formación Villa Mónica (800-900 Ma) se reconoce entre los componentes producidos durante el régimen eodiagenético, la presencia de caolinita, esmectita, clorita, ópalo y/o chert, goethita y hematita. Durante la mesodiagénesis se originaron interestratificados de illita-esmectita (con alta proporción de illita ~70%), cementos cuarzosos y recristalización de cementos silíceos generados en el régimen eodiagenético. A mayores profundidades se habría favorecido el crecimiento autigénico de illita, formación de estilolitas y de contactos suturados, disolución y reemplazo de cementos cuarzosos por hematita y calcita, como así también se infiere la transformación de caolinita a illita. Esto último indica que la unidad habría alcanzado el equilibrio a profundidades de 5 km o mayores y temperaturas elevadas (&gt;150ºC). Luego, esta unidad fue expuesta a la telodiagénesis, reflejado por la degradación de argilominerales, formación de caolinita y esmectita en los niveles pelíticos y por el desarrollo de una superficie cárstica en las dolomías del tope de la formación. La evolución de la diagénesis en las areniscas de la Formación Cerro Largo se inicia con productos de la eodiagénesis vinculada a un ambiente marino de condiciones reductoras con la formación de pirita, esmectita y glauconita. Luego, durante la mesodiagenésis el cemento predominante fue el cuarzoso, con fuente de sílice externa y la esmectita fue transformada en interestratificados de illita-esmectita (con moderada esmectita ~50-40%), sumado al crecimiento autigénico de illita. Tales evidencias permiten inferir que esta unidad habría alcanzado un régimen meosodiagenétco profundo (<4 km de profundidad y temperatura <120ºC), aunque menor que el observado en la unidad subyacente. La posterior exhumación de estas rocas favoreció las alteraciones telodiagenéticas como la caolinitización, formación de esmectita y degradación de los interestratificados de illita-esmectita. El cuarzo fue disuelto y reemplazado por una asociación de cementos de hematita-goethita-calcedonia, vinculados, desde el punto de vista diagenético, a condiciones subsuperficiales. En las pelitas de la Formación Olavarría los rasgos diagenéticos indican que las mismas fueron afectadas por una mesodiagénesis temprana, donde se destaca el crecimiento autigénico de illita en la matriz arcillosa. Se observa también la preservación de rasgos eodiagenéticos, como escasa deformación de la matriz, predominio de fábrica flotante (abierta), formación de concreciones de calcita de posible origen microbiano, concreciones estratales multiepisódicas de hematita y venillas de ópalo. Este estudio permite señalar la existencia de una marcada diferencia en el grado de enterramiento y modificación diagenética registrados en las facies silicoclásticas inferiores con respecto a las suprayacentes del Grupo Sierras Bayas, cuyo espesor total es actualmente inferior a los 200 m. Por lo tanto, se sugiere que la sección inferior habría sido exhumada y fuertemente erodada con la pérdida de parte del registro rocoso, por lo que la discordancia que las separa representaría un lapso temporal importante.<hr/>The Neoproterozoic sedimentary sequences of the Tandilia System Basin are represented in the Olavarría area by interstratified shallow marine, silicoclastic and carbonate units comprising, from oldest to youngest, for the Villa Mónica, Cerro Largo and Olavarría formations of the Sierras Bayas Group (Fig. 1). These almost undeformed and unmetamorphosed siliciclastics sedimentary units allowed studying the different diagenetic regimes due to the preservation of postdepositional features and, also, of many depositional and eodiagenetic ones. In the study area de Sierras Bayas Group is composed of the Villa Mónica (conglomerates, sandstones, shales and dolostones), Cerro Largo (chert breccias, quartz-sandstones and siltstones), Olavarría (mudstones) and Loma Negra (limestones) Formations with a thickness up to 185 m (Figs. 2 and 3; Iñiguez, et al., 1989; Poiré, 1987a, 1993; Gómez Peral, 2008). The siliciclastics facies of the Sierras Bayas Group are composed of conglomerates, sandstones and mudstones, represented in the basal section of the Villa Mónica Formation and the Cerro Largo and Olavarría formations (Fig. 4). The objective of the present study was to reveal the sequence of diagenetic processes represented in the silicoclastic facies from a detailed petrological analysis. It included the recognition of the allogenic and authigenic components, the characterization of the diagenetic microfabrics (chemical and compactational), the definition of microfacies and the interpretation of the postdepositional burial history from the determination of paragenetic sequences. Besides, a revision of the classical depositional facies chart was performed. Considering that different factors may influence the diagenesis of sediments at different times during its evolution (Kantorowicz, 1985), the general controls on diagenesis include the depositional mineralogy, the depositional-water chemistry, the change in pore fluid composition, the inferred burial pressures and the temperatures and subaerial exposure. From the lithofacial analisys we remark the presence of some facies with potential glacial origin here described as laminated gravelly mudstone (Fig. 5; facies FGh) in the basal section of the Villa Mónica Formation and the "Colombo" diamictite (Fig. 6; Ds) in the base of the Cerro Largo Formation. Other interesting level is present in the base of the Olavarría Formation where we recognized an intraformational conglomerate (Fig. 6; facies MCh) likely associated with a volcanic arc affinity by geochemical data (Zimmerman et al., 2011). The quartz-arkosic facies of the lower section of the Villa Mónica Formation can be separated in three petrofacies according with its detrital mineralogy: i) subarkosic sandstones (arenites and wackes) at the base; ii) lithic and sublithic sandstones rich in polycrystalline quartz (arenites and wackes) in the middle part and iii) quartz sandstones in the upper section (Table 1 and Fig. 7). A detailed analysis of the microfacies allowed the interpretation of the interaction of diagenetic processes with sediments during an important geological time interval (Tables 2, 3 and Figs. 8, 9 and 10). In the same sense, the identification of diagenetic facies permitted the differentiation of sections of the succession characterized by different diagenetic modifications. Clay minerals were analyzed since illite crystallinity, identification of interestratified illite/smectite and its smectite proportion help to assess the relative depth of burial reached for these sedimentary rocks. The main detrital and authigenic components, as well as the definition of their microstructures, allowed identifying 25 microfacies in the silicoclastic section of the Villa Mónica Formation (Table 2) and the interpretation of the evolution of the processes occurred during the different diagenetic regimens. The eodiagenetic regime involved degradation of K-feldspars to kaolinite, smectite, cementation with opal and hematite, as the result of the interaction with meteoric waters between surface and shallow burial depths. Subsequently, basinal brines controlled the diagenetic evolution of the sandstones and resulted in the initial transformation of smectite into interestratified illite-smectite, precipitation of quartz overgrowths, and recrystallization of siliceous cements. During further burial the most important alterations included transformation of interestratified illite-smectite in ordered type (R1) containing 70-80% of illite in relation with authigenic growth of illite; scarce authigenic chlorite as replacement of biotite and amphibole, illitization of kaolinite (inferred), pervasive stylolitization added to the formation of sutured contact grains, and later with hematite replacements and late calcite cements. All these features indicate an equilibrium of the fluids with very deep burial and high temperatures (Fig. 11; &gt; 5 km and &gt;150ºC). An important uplift is registered related to a period of intense erosion and weathering with the generation of a karstic surface on top of dolostones of the Villa Mónica Formation which constitutes a telodiagenetic surface. This important unconformity was situated in 595 Ma according to paleomagnetic studies, (Rapallini et al., 2008) and was characterized by dedolomitization, intense dissolution and precipitation of goethite, hematite, chert and calcite, which produce a typical reddish to pink coloration (Gómez Peral, 2008). Sandstones of the Cerro Largo Formation were divided in two petrofacies (Table 1; Fig. 7); the quartzose lower one is mostly composed of monocrystalline quartz with low proportion of matrix (< 10%), very scarce policrystaline quartz, chert and mudstone intraclasts. The analysis of detrital and diagenetic mineralogy, and the different microstructures allowed the recognition of 5 microfacies (Table 3). The diagenetic evolution of this unit began with eodiagenetic processes related to the formation of euhedral pyrite, infiltration of smectite, authigenic glauconite and chert cementation. The most important mesodiagenetic modifications included quartz cementation, which complete the pore-filling, followed of partial dissolution of quartz and replacement by subhedral hematitic cement. Progressive burial drove the transformation of detrital and eodiagenetic smectite, first into poorly and then into better-ordered, mixed-layer illite/ smectite (I/S). In the Cerro Largo Formation the I/S reaches a low ordered (R0) containing near of 40-50% of illite, added to the authigenic generation of illite with crystallinity index typical of diagenetic zone. Furthermore, these features can be related to a middle-deep mesodiagenesis (Fig. 11; < 4 km and < 120°C). However, this process was restricted to intercalated mudstones with clayminerals. Sparse stylolites and tangential to planar grain contacts are the most conspicuous chemical compaction indicators. Uplift and incursion of meteoric waters constrained the telodiagenetic alterations including kaolinitization and smectite generation as well as degradation of mixed-layer illite/smectite which increment their smectite proportion. One prominent telodiagenetic products in this quartz-sandstones was the dissolution of quartz cements in presence of pervasive fluids and its replacement by hematite, goethite and chalcedony. In the mudstones levels the presence of a clay association of smectite, kaolinite and pyrofillite was related to an advanced argillic alteration due to possibly hydrothermal influence. One of the most important inferences was the recognition of different burial diagenetic histories in the siliciclastic units of the lower part of the Sierras Bayas Group (lower section if the Villa Mónica Formation) respect to the upper successions (Cerro Largo and Olavarría formations) which implies that the discordance on top of the Villa Mónica Formation could represent a long period of erosion and subaerial exposure. <![CDATA[Hydrochemistry of Mountainous Rivers (Sierras de Córdoba, Argentina): dissolved major elements]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000100003&lng=en&nrm=iso&tlng=en Las Sierras Pampeanas de Córdoba constituyen un sitio importante para el estudio de sistemas hídricos por su importancia socio-económico-cultural. Allí se originan redes de drenajes muy importantes que proveen agua potable a la población del este y del oeste de las sierras. Litológicamente el área de estudio está representada por granitoides del Batolito de Achala (cuenca alta y media), y por gneises y sedimentos modernos aguas abajo. El clima es semiárido con precipitaciones medias anuales de ~750 mm. La clasificación geoquímica de los ríos y arroyos estudiados indica que, en general, son aguas diluidas y bicarbonatadas-mixtas a sódico-potásicas, con algunos ejemplos sin especie iónica dominante. El origen de los solutos está controlado por dos factores principales: las precipitaciones y la incipiente meteorización. La señal química de las precipitaciones prevalece en las cabeceras de los ríos, lo cual es notorio al analizar las señales de iones tales como Ca2+ y Cl-. El resto de los iones evidencian un aumento en las concentraciones en arroyos y vertientes de altura. La variación estacional de las precipitaciones ejerce un control principalmente sobre los elementos mayoritarios, cuyas concentraciones se diluyen en épocas húmedas y se incrementan durante el período seco, bajo condiciones de caudal de base. La dinámica de los iones mayoritarios se basa en los procesos geoquímicos que controlan el transporte de los mismos. Espacialmente se verifica un aumento en sus concentraciones (en especial del Ca2+) aguas abajo al igual que en el resto de los parámetros físico-químicos provenientes de la meteorización congruente e incongruente de los minerales presentes en el área de estudio. El modelo geoquímico propuesto (PHREEQC) para estos sistemas en particular evidencia que los principales procesos que explican la evolución química del agua en esta cuenca son la disolución y/o hidrólisis de minerales primarios presentes en las rocas aflorantes, tales como muscovita, oligoclasa, calcita, biotita y yeso y la formación de minerales secundarios tales como illita y caolinita. Para que se produzca esta meteorización química se consumen 4.10-3 moles de CO2 por litro de agua. Debido a la homogeneidad litológica y climática de las zonas serranas de la región de las Sierras Pampeanas, los resultados de este trabajo pueden ser extrapolados a la mayoría de los sistemas hídricos de la región, además de contribuir al conocimiento de la geoquímica de elementos mayoritarios en sistemas hídricos de alta montaña de otras regiones graníticas en el mundo.<hr/>The Sierras Pampeanas of Córdoba, Argentina, is an interesting locality to study the geochemistry of mountainous river systems, both for their socio-economic and cultural significance and for the diversity of streams and rivers that drain its slopes. Several important drainage networks in Córdoba Province have their headwaters in these ranges. In this work, the major chemical composition of water collected from four hydrological catchments is analyzed, with the aim of determining the sources of solutes, the geochemical processes that control their basin's dynamics and the influence of climatic conditions on the seasonal variation of elemental concentrations. Four drainage basins were selected for this study. All basins are classified as 5th order in Horton's classification (1945) (modified by Stralher, 1987) and their mean slopes vary between ~5%, in the eastern flank, and more than 9% on the range's western side. The study area is located between 31° 30' - 32° 00'S and between 64° 30' and 65° 10'W. Maximum altitude is about 2,400 m a.s.l. (Fig. 1). Dominant rocks are granitoids of the Achala Batholith that crop out in the upper and middle parts of the study basins, while gneisses and modern sediments dominate in the lower reaches. Climate is semiarid; mean rainfall reaches about 750 mm per year. The atmospheric input chemical signature (pluvial and snow precipitation) was analyzed and compared with that of springs and 1st order streams (Fig. 3) using an upper continental crust (UCC, McLennan, 2001) normalized diagram. The concentration of dissolved major and trace elements is of the order of 10³ to 10(7) times lower than the mean Upper Continental Crust. According to the observed patterns, atmospheric and spring chemical signals are very similar; however, elements such as Si, Na+ and in less importance Al+3 and Mg+2 are more enriched in springs and 1st order streams than in precipitations. The latter suggests an important contribution to water discharges in these small hydrological systems from rain/snow water stored in fractures and pores where long residence time allows silicate hydrolysis and the consequent release of more insoluble elements to the water, such as Al+3. Decreasing concentration of K+ in springs and streams is likely due to its incorporation into the illite lattice. Dissolved Ca2+ is the only major cation that keeps the atmospheric signal. In figure 4 river samples were divided according to their Horton's order and the major ionic composition was normalized to the corresponding average values in the precipitation. As seen in the diagram, all patterns show the same trend and, as usual, elemental concentrations increase in the flow direction, evidencing solute contribution by mineral weathering. Most elements are more concentrated in superficial waters with the exception of K+, which remains associated to clay minerals, and Cl-, whose concentration keeps constant, except in 5th order rivers. Increasing elemental concentration downflow is mainly due to silicate weathering and carbonates dissolution. The latter is probably associated with atmospheric dust and disseminated calcite in regolith and soils from the study area. Mountainous rivers and streams are diluted due to the short water-rock contact time and also because soils and sediment accumulations are shallow. In the study area, total dissolved solids (TDS) vary between 10 and 60 mg l-1, which implies a shared atmospheric and weathering control (Gibbs, 1970). River waters draining crystalline rocks show electrical conductivity values from <20 to ~150 µS cm-1, with a mean of 65.8 µS cm-1. On the other hand, waters in contact with unconsolidated sedimentary material show moderate electrical conductivity values (483 µS cm-1). Waters are slightly acid to alkaline; with pH values that range between 5.58 and 8.80 with a mean of 7.45 ± 0.69 (mean ± SD). When rainfall water is in equilibrium with atmospheric CO2, it exhibits a pH of about 5.6 (Drever, 1997). During times of decreased rainfall, the increased concentration of atmospheric dust is responsible for the observable increased pH of rainfall, particularly during the rain events that occur at the beginning of the humid cycle (García et al., 2007; García et al., 2009). As usual, electrical conductivity increases downstream with the lowest values ~18 µS cm-1, in the headwaters. Alkalinity (HCO3)- ranges between 0.058 and 2.412 meq l-1 (3.5 - 147.1 mg l-1) with a mean of 0.37 meq l-1 (22.2 mg l-1) for the whole studied area. According to their major chemical composition, rivers and streams waters are of the bicarbonate-mixed, and bicarbonate-sodium-potassium type (Piper, 1944), but some samples show no dominant anionic type. Seasonal variations of ionic concentrations were analyzed throughout the three different sampling periods. Highest concentrations were always measured under base flow conditions while the lowest were registered at maximum discharges. Elevated ionic concentrations during base discharge condition are due to the combination of several factors, principally the higher contribution of groundwater to river discharge, and in less importance the increased evapotranspiration, and increased fallout of dust accumulated in the atmosphere (aerosols). To quantify chemical weathering, a geochemical model was employed as an example of mineral and gas phases dissolved, hydrolyzed or precipitated. By means of PHREEQC 2.13 inverse modeling, the chemical weathering reactions were simulated in the study area. Two samples collected in the Panaholma river were used to run the model: one of the samples correspond to the basin's headwaters (initial solution; 1st order, 2,179 m a.s.l.) and the other to the river's lower reaches (in the limit of Achala granite; final solution; 5th order, 1,145 m a.s.l.). Modeling results are included in Table 2, detailed intervening phases are listed, mol l-1 H2O transferred, and percentages of each one. PHREEQC inverse models developed using water chemical data showed that: a) weathering transfers ~1 mol l-1 in granitic and steep areas with semi-arid climatic conditions (e.g., Panaholma River); b) muscovite and oligoclase are the major supplier of solutes, followed by calcite, biotite and gypsum; c) illite and kaolinite (in order of importance) are the main products of weathering; d) CO2 consumption by weathering processes reaches 4 mmol kg-1 H2O; and e) CO2 accounts for nearly 40% of all the species involved in the weathering reactions occurring at the Panaholma drainage basin. All these results indicate incipient weathering, in agreement with what it is known in regions with semi arid conditions. Moreover, these results allows to infer a moderate chemical weathering rate for the whole granitic region in the Sierras de Córdoba (Argentina) and leads to extrapolate these results to other similar areas and compare them with other granitic regions. <![CDATA[Dr. Eduardo Aldo Musacchio (1940-2011)]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000100004&lng=en&nrm=iso&tlng=en Las Sierras Pampeanas de Córdoba constituyen un sitio importante para el estudio de sistemas hídricos por su importancia socio-económico-cultural. Allí se originan redes de drenajes muy importantes que proveen agua potable a la población del este y del oeste de las sierras. Litológicamente el área de estudio está representada por granitoides del Batolito de Achala (cuenca alta y media), y por gneises y sedimentos modernos aguas abajo. El clima es semiárido con precipitaciones medias anuales de ~750 mm. La clasificación geoquímica de los ríos y arroyos estudiados indica que, en general, son aguas diluidas y bicarbonatadas-mixtas a sódico-potásicas, con algunos ejemplos sin especie iónica dominante. El origen de los solutos está controlado por dos factores principales: las precipitaciones y la incipiente meteorización. La señal química de las precipitaciones prevalece en las cabeceras de los ríos, lo cual es notorio al analizar las señales de iones tales como Ca2+ y Cl-. El resto de los iones evidencian un aumento en las concentraciones en arroyos y vertientes de altura. La variación estacional de las precipitaciones ejerce un control principalmente sobre los elementos mayoritarios, cuyas concentraciones se diluyen en épocas húmedas y se incrementan durante el período seco, bajo condiciones de caudal de base. La dinámica de los iones mayoritarios se basa en los procesos geoquímicos que controlan el transporte de los mismos. Espacialmente se verifica un aumento en sus concentraciones (en especial del Ca2+) aguas abajo al igual que en el resto de los parámetros físico-químicos provenientes de la meteorización congruente e incongruente de los minerales presentes en el área de estudio. El modelo geoquímico propuesto (PHREEQC) para estos sistemas en particular evidencia que los principales procesos que explican la evolución química del agua en esta cuenca son la disolución y/o hidrólisis de minerales primarios presentes en las rocas aflorantes, tales como muscovita, oligoclasa, calcita, biotita y yeso y la formación de minerales secundarios tales como illita y caolinita. Para que se produzca esta meteorización química se consumen 4.10-3 moles de CO2 por litro de agua. Debido a la homogeneidad litológica y climática de las zonas serranas de la región de las Sierras Pampeanas, los resultados de este trabajo pueden ser extrapolados a la mayoría de los sistemas hídricos de la región, además de contribuir al conocimiento de la geoquímica de elementos mayoritarios en sistemas hídricos de alta montaña de otras regiones graníticas en el mundo.<hr/>The Sierras Pampeanas of Córdoba, Argentina, is an interesting locality to study the geochemistry of mountainous river systems, both for their socio-economic and cultural significance and for the diversity of streams and rivers that drain its slopes. Several important drainage networks in Córdoba Province have their headwaters in these ranges. In this work, the major chemical composition of water collected from four hydrological catchments is analyzed, with the aim of determining the sources of solutes, the geochemical processes that control their basin's dynamics and the influence of climatic conditions on the seasonal variation of elemental concentrations. Four drainage basins were selected for this study. All basins are classified as 5th order in Horton's classification (1945) (modified by Stralher, 1987) and their mean slopes vary between ~5%, in the eastern flank, and more than 9% on the range's western side. The study area is located between 31° 30' - 32° 00'S and between 64° 30' and 65° 10'W. Maximum altitude is about 2,400 m a.s.l. (Fig. 1). Dominant rocks are granitoids of the Achala Batholith that crop out in the upper and middle parts of the study basins, while gneisses and modern sediments dominate in the lower reaches. Climate is semiarid; mean rainfall reaches about 750 mm per year. The atmospheric input chemical signature (pluvial and snow precipitation) was analyzed and compared with that of springs and 1st order streams (Fig. 3) using an upper continental crust (UCC, McLennan, 2001) normalized diagram. The concentration of dissolved major and trace elements is of the order of 10³ to 10(7) times lower than the mean Upper Continental Crust. According to the observed patterns, atmospheric and spring chemical signals are very similar; however, elements such as Si, Na+ and in less importance Al+3 and Mg+2 are more enriched in springs and 1st order streams than in precipitations. The latter suggests an important contribution to water discharges in these small hydrological systems from rain/snow water stored in fractures and pores where long residence time allows silicate hydrolysis and the consequent release of more insoluble elements to the water, such as Al+3. Decreasing concentration of K+ in springs and streams is likely due to its incorporation into the illite lattice. Dissolved Ca2+ is the only major cation that keeps the atmospheric signal. In figure 4 river samples were divided according to their Horton's order and the major ionic composition was normalized to the corresponding average values in the precipitation. As seen in the diagram, all patterns show the same trend and, as usual, elemental concentrations increase in the flow direction, evidencing solute contribution by mineral weathering. Most elements are more concentrated in superficial waters with the exception of K+, which remains associated to clay minerals, and Cl-, whose concentration keeps constant, except in 5th order rivers. Increasing elemental concentration downflow is mainly due to silicate weathering and carbonates dissolution. The latter is probably associated with atmospheric dust and disseminated calcite in regolith and soils from the study area. Mountainous rivers and streams are diluted due to the short water-rock contact time and also because soils and sediment accumulations are shallow. In the study area, total dissolved solids (TDS) vary between 10 and 60 mg l-1, which implies a shared atmospheric and weathering control (Gibbs, 1970). River waters draining crystalline rocks show electrical conductivity values from <20 to ~150 µS cm-1, with a mean of 65.8 µS cm-1. On the other hand, waters in contact with unconsolidated sedimentary material show moderate electrical conductivity values (483 µS cm-1). Waters are slightly acid to alkaline; with pH values that range between 5.58 and 8.80 with a mean of 7.45 ± 0.69 (mean ± SD). When rainfall water is in equilibrium with atmospheric CO2, it exhibits a pH of about 5.6 (Drever, 1997). During times of decreased rainfall, the increased concentration of atmospheric dust is responsible for the observable increased pH of rainfall, particularly during the rain events that occur at the beginning of the humid cycle (García et al., 2007; García et al., 2009). As usual, electrical conductivity increases downstream with the lowest values ~18 µS cm-1, in the headwaters. Alkalinity (HCO3)- ranges between 0.058 and 2.412 meq l-1 (3.5 - 147.1 mg l-1) with a mean of 0.37 meq l-1 (22.2 mg l-1) for the whole studied area. According to their major chemical composition, rivers and streams waters are of the bicarbonate-mixed, and bicarbonate-sodium-potassium type (Piper, 1944), but some samples show no dominant anionic type. Seasonal variations of ionic concentrations were analyzed throughout the three different sampling periods. Highest concentrations were always measured under base flow conditions while the lowest were registered at maximum discharges. Elevated ionic concentrations during base discharge condition are due to the combination of several factors, principally the higher contribution of groundwater to river discharge, and in less importance the increased evapotranspiration, and increased fallout of dust accumulated in the atmosphere (aerosols). To quantify chemical weathering, a geochemical model was employed as an example of mineral and gas phases dissolved, hydrolyzed or precipitated. By means of PHREEQC 2.13 inverse modeling, the chemical weathering reactions were simulated in the study area. Two samples collected in the Panaholma river were used to run the model: one of the samples correspond to the basin's headwaters (initial solution; 1st order, 2,179 m a.s.l.) and the other to the river's lower reaches (in the limit of Achala granite; final solution; 5th order, 1,145 m a.s.l.). Modeling results are included in Table 2, detailed intervening phases are listed, mol l-1 H2O transferred, and percentages of each one. PHREEQC inverse models developed using water chemical data showed that: a) weathering transfers ~1 mol l-1 in granitic and steep areas with semi-arid climatic conditions (e.g., Panaholma River); b) muscovite and oligoclase are the major supplier of solutes, followed by calcite, biotite and gypsum; c) illite and kaolinite (in order of importance) are the main products of weathering; d) CO2 consumption by weathering processes reaches 4 mmol kg-1 H2O; and e) CO2 accounts for nearly 40% of all the species involved in the weathering reactions occurring at the Panaholma drainage basin. All these results indicate incipient weathering, in agreement with what it is known in regions with semi arid conditions. Moreover, these results allows to infer a moderate chemical weathering rate for the whole granitic region in the Sierras de Córdoba (Argentina) and leads to extrapolate these results to other similar areas and compare them with other granitic regions. <![CDATA[Tsunami vs storm origin for shell bed deposits in a lagoon enviroment: an example from the Upper Cretaceous of Southern Patagonia, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792011000100005&lng=en&nrm=iso&tlng=en The criteria by which the deposits of tsunamis are distiguished from other deposits, including storm surges, have been controversial for more than 10 years. The Mata Amarilla Formation of the lower Upper Cretaceous of Southern Patagonia has excellent outcrops that in its lower section, sedimentary and taphonomic characteristics suggest a tsunami origin. This paper presents details of these aspects, as well as a model of temporal stages that led to their deposition within a lagoon. The sediments are composed of alternating white sandstones and mudstones, with interbedded bioclastic accumulations in the lower section. The depositional environment was characterised by a lagoon bounded by shallow marine bars. These fine-grained sediments are sporadically interrupted by tsunami events represented by coquinas, bioclastic sands and shell pavements with allochthonous and autochthonous mollusk associations from freshwater and marine habitats. Some areas of the lagoon became exposed, thus enabling the development of vegetation on the substrate and pedogenic processes. Subsequently, a forced regression occurred when a fluvial system invaded the lagoon area, representing the beginning of the deposition of the middle section of the Mata Amarilla Formation.<hr/>Los criterios por los cuales los depósitos de los tsunamis son diferenciados de otros depósitos, incluidas las grandes tormentas, han sido motivo de controversia desde hace más de 10 años. La Formación Mata Amarilla del Cretácico Superior más bajo de la Patagonia Austral posee excelentes afloramientos en su sección inferior cuyas características sedimentológicas y tafonómicas permiten interpretarlos como originados por tsunamis. En este trabajo se presentan los detalles de sus características sedimentológicas y tafonómicas, así como un modelo de las etapas temporales de depositación en un ambiente albuférico. Los sedimentos están compuestos por la alternancia de arenicas blancas y fangolitas, con intercalaciones de acumulaciones bioclásticas en la parte inferior. El ambiente de sedimentación estuvo representado por una albufera de poca profundidad limitada por un sistema de barreras. Estos sedimentos de grano fino intercalan con depósitos asociados con el desarrollo de tsunamis, que incluyen coquinas, areniscas bioclásticas y pavimentos de valvas de moluscos con asociaciones de fauna tanto alóctonas (ambiente marino) como autóctonas (agua dulce). Algunas áreas de la albúfera quedaron expuestas, lo que permitió el desarrollo de vegetación sobre el sustrato y la generación de procesos pedogenéticos. Posteriormente, se produjo una regresión forzada cuando un sistema fluvial invadió la zona de la albufera, representando el inicio de la deposición de la sección media de la Formación Mata Amarilla.