Scielo RSS <![CDATA[Latin American journal of sedimentology and basin analysis]]> http://www.scielo.org.ar/rss.php?pid=1851-497920080001&lang=es vol. 15 num. 1 lang. es <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <![CDATA[Análisis Estratigráfico Secuencial de las Formaciones Huincul y Lisandro del Subgrupo Río Limay (Grupo Neuquén - Cretácico Tardío) en el Departamento El Cuy, Río Negro, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792008000100001&lng=es&nrm=iso&tlng=es Se estudió una sucesión sedimentaria que aflora en el sudeste de la Cuenca Neuquina, en el departamento El Cuy (Río Negro). La misma incluye a las formaciones Huincul y Lisandro que integran el Subgrupo Río Limay (Grupo Neuquén), y han sido asignadas al Cenomaniano- Turoniano. Se identificaron once litofacies de origen fluvial, tres litofacies de naturaleza eólica y dos litofacies volcaniclásticas. A partir de los agrupamientos en unidades genéticamente relacionadas se definieron siete elementos fluviales de intracanal, cuatro de planicie de inundación y cuatro eólicos. La distribución espacial y relaciones entre los elementos arquitecturales ha permitido reconocer sistemas fluviales agradantes de alta sinuosidad (SF-I y II), un sistema de abanico terminal (SAT) y campos de médanos barjanoides (SE). El análisis estratigráfico-secuencial permitió definir cinco Secuencias a partir de las relaciones espaciales y temporales de los elementos arquitecturales mayores y de la determinación de superficies estratigráficamente significativas. La base de la Formación Huincul está representada por el SF-I, caracterizado por un sistema de alta sinuosidad, con baja relación canales/planicie de inundación y abundantes depósitos de desbordes. Este tramo de la secuencia se interpreta como el Cortejo de Alta Acomodación (AA) de la Secuencia I (S-I). El sector cuspidal de la unidad está integrado por un sistema similar pero con una alta relación canal/planicie de inundación y constituye el Cortejo de Baja Acomodación (BA) de la Secuencia II (S-II). Se propone que la depositación de la Formación Huincul tuvo lugar bajo condiciones climáticas cálidas con un régimen de estacionalidad marcado. La Formación Lisandro se inicia con depósitos de un sistema distributario proximal de abanico terminal (SAT) que representan el Cortejo AA de S-II. De aquí en adelante es notoria la recurrencia de SE, caracterizando los Cortejos BA y una sucesión desde cuenca de inundación a distributarios proximales de SAT, integrando los Cortejos AA de las secuencias S-II, III, IV y V. La unidad se depositó bajo condiciones climáticas semiáridas permanentes. Las unidades estratigráficas estudiadas se consideran de carácter sinorogénico, con un control tectónico significativo en la definición del espacio de acomodación y un volcanismo principalmente activo durante la sedimentación de la Formación Huincul. La Formación Lisandro corresponde a un período de abrupto incremento en la tasa de subsidencia en la Cuenca Neuquina.<hr/>In the Cuy Department, Rio Negro Province (Fig. 1), there are outcrops of the Late Cretaceous continental deposits of the Subgroup Rio Limay (Fig. 2), in virtually continuous exposures that allow detail studies and exploration of the architecture of the sedimentary bodies. This contribution presents the results of the sedimentological study and sequential stratigraphic analysis for the depositional period of the Huincul and Lisandro formations (Subgroup Río Limay). The outcrops appear in the Planicie de Rentaría (Area A), with continuous sections longer than 3 km and Anfiteatro (Area B) with outcrops of more than 5 km (Fig. 1). In these areas we made detailed sedimentological profiles (Fig. 3), we described and interpreted lithofacies and architectural elements employing photograms and we defined stratigraphicaly significant discontinuities. We identified 11 fluvial lithofacies, 4 conglomerates (Gm, Gh, Gp and Gt), 5 sandstones (Sm, St., Sp, and Mr. Sh), and 2 pelitic (Fl and Fm), according to the nomenclature of Miall (1996). In addition, 2 volcaniclastic lithofacies, 1 primary pyroclastic lithofacies (Tlpm) and 1 volcaniclastic sineruptive lithofacies (TLpe) following criteria Mc Phie et al. (1993) were defined, and finally 3 eolic lithofacies (Spe, She and Sathya) (Fig. 4). These were grouped into 11 fluvial architectural elements and 4 eolic (Fig. 5). We have recognized channel elements (CHI) and CHII), intrachannel (LA, DAI, DAII, SBI and SBII) and floodplain (CRI, CRII, CS and OF). The eolic architectural elements include dunes (ED) and interdunes (DI, and WI FI). The paleoenvironments sedimentary include river systems, terminal fans and dune fields. The Sistema Fluvial I (SF-I) is integrated by LA, DAI, DAII, SBI, CHI, CRI, CRII and OF (Fig. 6). The meanders loops show, in most cases, complete filling sequences in which records of migration of large sand dunes are preserved which retain topsets deposits and the scrolls their undulating tops. The channel belts are separated by potent floodplain deposits, with frequent intercalation of levels of overflow both channelized or in mantle, and show a multilateral persistence in potent sections of the column. The crevasse splay exhibits paleosoils with low levels of development, frequent presence of gleyzed horizons and high degree of bioturbation. The general characteristics of the S-FI are proper of aggrading fluvial systems of high sinuosity. The Sistema Fluvial II (SF-II) is characterized by LA, DAI, DAII, SBI and to a lesser proportion, by CRI and OF (Fig. 6). The meanders loops often show preservation of the channel fill by lateral accretion with complete successions. Also the dunes preserve topsets deposits and the scrolls the original topography (Fig. 7). The frequency of conservation of filling phase of the abandoned channel with strong bioturbation and development gleyzed horizons suggests frequent avulsion by strangulation. The preservation of the floodplain is low, and it is characterized by intense bioturbation, and a high volcaniclastic participation in some reaches. We consider that the fluvial system model is of high sinuosity highly aggrading. The terminal fan system (Sistema Abanico Terminal) (TSS), characteristic of semiarid regions with strong seasonal climatology, is represented by CHII, CS or SBII and OF (Fig. 6). These constitute flooding basin deposits (CI), simple multiepisodic channel fillings corresponding to middle sections of the distributary plain and simple channels with development of small transversal cross bars and multilateral relations of distal distributary plain. The aeolian system (Sistema Eolico) (IS) consists of SD, DI, WD and FD. The spatial distribution of dunes and interdunes, the high dispersion of paleocurrents with a principal mode at the avalanche deposits and multiple stabilization surfaces suggest the development of a field of barchan dunes. In the element WI, we identified ferric and mottled halos and bioturbations that indicate the presence of paleosoils with gleyzed horizons (Retallack, 1976, 1990) which and characterize environments with good oxygenation. According to the temporal and spatial arrangement of the different paleoenvironments, their evolutionary trend and the analysis of significant discontinuity surfaces, we produced the stratigraphic sequence for the Subgroup Rio Limay (Fig. 8). The sequence starts with the Secuencia I, corresponding to the base of the Huincul Fm and is represented by SF-I, defined by belts of potent channels, isolated in floodplain deposits and with the development of overflow fans under conditions of rapid aggradation. The avulsion by overflow crevasse splay was the control mechanism of the system. The development of composite paleosoils in distal segments of the plain indicates the existence of long periods during which the channel band remained relatively stable poitions. The incorporation of volcaniclastic materials to fluvial deposits claims a volcanic activity close in time. The geometry and continuity of the sandy bodies and its proportion in relation to the floodplain deposits suggests periods of low frequency of avulsion/subsidence rate. The large-scale architecture observed is the equivalent to a High Accommodation System Track (Cortejo) (AA) with a ratio A/S close to 1 associated with a gradual decline in the base level under conditions of high sediment supply. A net surface (Discontinuidad D-IS-IS) is the basis of the sequence II, marked by an abrupt change in the fluvial architecture river (SF-II). The mechanism of lateral migration was by strangulation and abrupt cut of the bends. Only underdeveloped paleosoils in the abandoned channel fillings and the little preservation of the floodplain deposits are recognized. The stacking of channel belts signs out a reduction in the accommodation space. The top layer of Huincul Fm is interpreted as Lower Accommodation System Track (Cortejo) (BA). The Discontinuity IIC (D-IIC) marks the base of Lisandro Fm, which is an surface of fluvial flooding defining the expansion of a terminal fan system associated with an abrupt increase in the accommodation space and rapid progradation of the facies of an environment of intermediate distributary plain. This stretch of the Sequence is interpreted as the High Accommodation System Track (Cortejo) (AA) of the Sequence II (S-II) was associated with a period of high subsidence. The deposits of terminal fans are covered by a wet eolian system (IS). The change in the depositional environment marks the beginning of the Sequence III (SIII), limited at the base by D-IIIS. The development of a field of parabolic dunes associated with environments dry, wet and flooded interdune indicates a gradual rise in the water table that, eventually, was above the depositional surface generating shallow lagoons. The shortterm climatic changes related with an increase in precipitation and the relative position of the water table controlled the development stabilization surfaces and the growth and migration of aeolian forms. We identified two supersurfaces (D-IV and DV, Fig. 18) which may be assigned to periods with an increase in the supply or availability of sediment climatically conditioned. Periodically, the interdune corridors were overrun by ephemeral streams associated with the flood basin environments of SAT during flooding events. All deposits, characterized by a strong stacking of the eolian successions are interpreted as the Cortejo BA from the Sequence III. A new fluvial flood event marked by the Discontinuity VIC, is represented by facies flood basin, a distributary plain with the development of intertwined channels showing evidences of the SAT progradation. This sector is interpreted as the (Cortejo) AA System Track of S-III associated with a period of renewed subsidence. D-VIIS limits the base of the S-IV whose (Cortejo) BA System Track is similar to the previous Sequence and includes 3 supersurfaces (D-D-VIIIIX / X). The expansion of the flooding basin facies of a limited terminal fan at the base by D-XIC represents the base of the (Cortejo) AA System Track of the Sequence IV. Tabular units associated with a distal distributary plain that replaced towards the top by thick channels of low sinuosity close to the backbone of the system represent an progradation event of the SAT. The beginning of the Sequence V is marked D-XIIS associated with the development of a new SE. The architectural design of large scale is similar to that of the (Cortejos) BA System Tracks of the Sequences III and IV and includes supersurfaces D-D-XIII and XIV. Something similar occurs with the surface of fluvial flooding (D-XVC) on which we identified a prograding succession of SAT. The characteristics of the systems involved in the Lisandro Fm suggest a climate change to semi-arid conditions and low gradient in the depositional environment. The sequential stratigraphic analysis of the formations Huincul and Lisandro suggests that the deposits are sinorogénic and the different pulses of subsidence are registered in the (Cortejos) AA and BA System Tracks defined in this study. <![CDATA[Petrográfica y Geoquímica de la dolomía hospedante de una mineralización de Zn y Pb. Puesto Gregor, Neuquén, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792008000100002&lng=es&nrm=iso&tlng=es En la Cuenca Neuquina (Argentina) y asociada a la Formación Lajas (Jurásico) se mencionó por primera vez una mineralización de esfalerita y galena asociada a carbonatos. En el área del yacimiento Puesto Gregor, ésta Formación está constituida por una secuencia clástico carbonática. Para caracterizar el ambiente de depositación de la dolomía hospedante de la mineralización de Zn y Pb se realizaron estudios petrográficos y geoquímicos. Entre los últimos los más utilizados son los análisis e interpretación de isótopos estables (O, C), elementos mayoritarios y trazas e inclusiones fluidas. En el horizonte dolomítico de 0,90 m de espesor y 300 m de corrida se distinguen petrográficamente dos tipos de dolomita: el primer tipo es penetrativo, sin preservación de la fábrica sedimentaria original y es anterior a la mineralización, el segundo tipo está vinculado a la precipitación de sulfuros y consiste en una dolomita de tipo silla de montar (saddle). El estudio de las inclusiones fluidas en la dolomita penetrativa indica la presencia de fluidos orgánicos y fluidos acuosos. Determinaciones microtermométricas sobre inclusiones fluidas acuosas revelaron temperaturas de 140º C. Geoquímicamente la capa presenta proporciones casi estequiométricas y promedios de CaCO3 y MgCO3 iguales al 52 y 48% molar, respectivamente. En cuanto a los elementos trazas, el Sr varió entre 79 y 159 ppm y el Na entre 74 y 225 ppm. Los contenidos de Mn y Fe son mucho más altos que los valores determinados para estos elementos en rocas carbonáticas. El contenido en ETR es bajo y los diagramas de ETR muestran una anomalía negativa de Ce y de Eu. Se determinaron valores de isótopos de C y O. Los valores de ä13 C (VPDB) entre -2,9 y -9 0/00 no son coherentes con los datos informados para calizas marinas de edad jurásica mientras que los valores de ä18O(VPDB) entre -2,6 y -4 0/00 coinciden con los valores informados para carbonatos de igual edad. Los datos de ä18O indican que la interacción entre el fluido y la roca de caja fue baja. Los valores del ä18O del fluido en equilibrio con la dolomía a temperaturas de 140ºC indican que este fluido corresponde a una salmuera de cuenca. Por otro lado, los isótopos de carbono indican un aporte de carbono procedente de la diagénesis de la materia orgánica causada por un aumento de temperatura vinculada al soterramiento de la cuenca. Los resultados petrográficos y geoquímicos son consistentes con la dolomitización de un carbonato de origen marino que sufrió soterramiento.<hr/>In Neuquen Basin, Argentina, a Zn-Pb mineralization was first reported by Garrido et al. (2000). The ore occurs in a carbonatic level located in Puesto Gregor, 50 km SSE from the city of Zapala at 39°11'34'' S, 69° 59'18'' W (Fig. 1). The hosting bed, a dolostone, belongs to a carbonatic-siliciclastic sequence of Lajas Formation, which is part of the Cuyo Group, Jurassic age. In the mineralization of Pb-Zn deposits associated to dolostones, the fluids that were involved in the ore precipitation process were also related to the dolomitization of the carbonatic rock (Warren, 2000). In this contribution, field, petrographic and geochemical studies were carried out in order to determine the temperature and composition of the dolimitizing fluids. The obtained results were then compared to those obtained from Carbon and Oxygen isotopes (Garrido et al., 2001) to discuss the dolimitization process. The mineralized bed, 0.90 m thick, outcrops for about 300 m along the strike (W-E) and 60 m in the dip direction (Fig. 2). This bed pinches out toward the east and toward the west it is no longer visible, it is cover by scours. Petrographic studies determined that the host rock is a dolostone with a breccia texture that becomes more siliciclastic towards the east grading thus to a fine sandstone with carbonatic cement. The hypogenic mineralization, mainly sphalerite, low quantities of galena, pyrite and marcasite is found within the small fractures. Some ghosts of fossils are still visible, but pervasive dolomitization characterizes the horizon. Two distinct dolomites are recognized by crosscutting relationships: a fine to medium grained crystalline dolomite, and a coarse grained crystalline dolomite related to the mineralization. The fomer shows dark-orange and white crystals which occur as patches or partially filled vugs. These crystals are 120-400 mm in size and exhibit subhedral to anhedral shapes (Fig. 3). According to Sibley and Gregg (1987), the texture is no planar -a- unimodal to polimodal. The latter dolomite presents well developed crystals (> 5 mm); they are translucent with pink color and pearl shine and have crystal faces that look like a pavement and is referred as "saddle" dolomite according to Radke and Mathis (1980). This "saddle" dolomite is found into dissolution cavities or as clusters of crystals located on the wall fractures; it is always associated to the mineralization. Chemical analysis of major, traces and rare earth elements are homogeneous throughout the bed. Mean values are 15% MgO, 29,66% CaO and 40,43 % CO2, with high MnO and Fe2O3 contents. The molar percentages of CaCO3 and MgCO3 indicate near stoichiometric ratio (52% and 48%) with a light excess of Ca (Table 1, Fig. 4). The trace elements Sr, Na, Fe and Mn are used to constrain dolomite evolution. Sr values varies from 79 to 159 ppm and Na from 74 to 225 ppm; Mn and Fe contents are higher than the values determined for carbonatic rocks (Turekian and Wedepohl, 1961). ÓREE and LREE contents are low, and the diagram normalized to chondrite shows a negative anomaly of Eu and a great negative anomaly of Ce. The 13C (VPDB) and 18O (VPDB) values vary from -2,9 to -9(0)/00 and from -2,6 to -4 0/00 respectively (Table 2). The 13C are incoherent with the data recorded for Jurassic marine carbonates (near 0 0/00) while 18O values can be correlated with carbonates of the same age (Veizer et al., 1999). Petrography and chemical analysis allow characterizing the depositional environment of the Zn- Pb mineralized dolostone. The xenotopic texture of the dolomite with no planar crystals, gives evidence that the temperatures of deposition should have been higher than 50-60°C (Gregg and Sibley, 1984). On the other hand, the chemical composition, near ideal dolomites (stoichiometric ratio), indicates slow crystallization at high temperature (Morrow, 1982). Morover, the destructive fabric and the homogeneous composition suggest a high temperature dolomitization (Machel, 2004). Trace element values, mainly Na and Sr, agree with burial dolomites, as well as the fluid inclusions reported for these samples by Cesaretti et al. (2002). The negative Ce anomaly indicates that these rocks were formed in a marine environment. Two different processes of carbonate precipitation can produce negative Ce anomaly (Möller, 1989; Bau and Möller, 1992): deposition from seawater or from hydrothermal solutions equilibrated with highly oxydized sediments. The latter is discarded because of the presence of framboidal pyrite and organic matter, which, along with the Eu negative anomaly indicates that the dolimitization were generated under euxinic conditions. This dolostone is in contact with anoxic mudstones (Los Molles Formation, Cuyo Group). Petrographic and geochemical criteria reflect that the dolomitization were caused by normal or modified sea water in a burial environment at temperatures above 140ºC. In burial or altered dolostones, the oxygen isotopes reflect temperature of precipitation and isotope composition of the dolomitizing fluids. The oxygen isotope values of this dolomitized bed are compatible with the isotope composition of carbonates precipitated from sea water at 25°C. The narrow range in the obtained values indicates that there was no influence of meteoric water during this process (Allan and Mathews, 1982). The homogeneous values of 18O isotope suggest that the physic-chemical conditions remained constant during dolomitization, what is in agreement with the textural and geochemical homogeneity found in the study samples. The 18O isotope values of a fluid equilibrated with carbonate at 140°C indicate that the fluid belongs to a basinal fluid. The 13C isotopes reflect an organic origin for the carbon. This carbon came from the diagenesis of organic matter caused by an increase in temperature during the burial of the basin (Garrido et al., 2001; Cesaretti et al., 2002). In contrast with other MVT deposits of the world, in Puesto Gregor, the dolomitization was slow process acting at high temperatures, what has been confirmed by the homogeneity of the fabric and the narrow range in the isotope and trace elements composition. These conditions were reached during burial of the basin where the rocks interact with the basin fluids associated to the ore minerals. <![CDATA[Glacial Deposits in the Río del Peñón Formation (Late Carboniferous), Río Blanco Basin, Northwestern Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792008000100003&lng=es&nrm=iso&tlng=es En la Cuenca Neuquina (Argentina) y asociada a la Formación Lajas (Jurásico) se mencionó por primera vez una mineralización de esfalerita y galena asociada a carbonatos. En el área del yacimiento Puesto Gregor, ésta Formación está constituida por una secuencia clástico carbonática. Para caracterizar el ambiente de depositación de la dolomía hospedante de la mineralización de Zn y Pb se realizaron estudios petrográficos y geoquímicos. Entre los últimos los más utilizados son los análisis e interpretación de isótopos estables (O, C), elementos mayoritarios y trazas e inclusiones fluidas. En el horizonte dolomítico de 0,90 m de espesor y 300 m de corrida se distinguen petrográficamente dos tipos de dolomita: el primer tipo es penetrativo, sin preservación de la fábrica sedimentaria original y es anterior a la mineralización, el segundo tipo está vinculado a la precipitación de sulfuros y consiste en una dolomita de tipo silla de montar (saddle). El estudio de las inclusiones fluidas en la dolomita penetrativa indica la presencia de fluidos orgánicos y fluidos acuosos. Determinaciones microtermométricas sobre inclusiones fluidas acuosas revelaron temperaturas de 140º C. Geoquímicamente la capa presenta proporciones casi estequiométricas y promedios de CaCO3 y MgCO3 iguales al 52 y 48% molar, respectivamente. En cuanto a los elementos trazas, el Sr varió entre 79 y 159 ppm y el Na entre 74 y 225 ppm. Los contenidos de Mn y Fe son mucho más altos que los valores determinados para estos elementos en rocas carbonáticas. El contenido en ETR es bajo y los diagramas de ETR muestran una anomalía negativa de Ce y de Eu. Se determinaron valores de isótopos de C y O. Los valores de ä13 C (VPDB) entre -2,9 y -9 0/00 no son coherentes con los datos informados para calizas marinas de edad jurásica mientras que los valores de ä18O(VPDB) entre -2,6 y -4 0/00 coinciden con los valores informados para carbonatos de igual edad. Los datos de ä18O indican que la interacción entre el fluido y la roca de caja fue baja. Los valores del ä18O del fluido en equilibrio con la dolomía a temperaturas de 140ºC indican que este fluido corresponde a una salmuera de cuenca. Por otro lado, los isótopos de carbono indican un aporte de carbono procedente de la diagénesis de la materia orgánica causada por un aumento de temperatura vinculada al soterramiento de la cuenca. Los resultados petrográficos y geoquímicos son consistentes con la dolomitización de un carbonato de origen marino que sufrió soterramiento.<hr/>In Neuquen Basin, Argentina, a Zn-Pb mineralization was first reported by Garrido et al. (2000). The ore occurs in a carbonatic level located in Puesto Gregor, 50 km SSE from the city of Zapala at 39°11'34'' S, 69° 59'18'' W (Fig. 1). The hosting bed, a dolostone, belongs to a carbonatic-siliciclastic sequence of Lajas Formation, which is part of the Cuyo Group, Jurassic age. In the mineralization of Pb-Zn deposits associated to dolostones, the fluids that were involved in the ore precipitation process were also related to the dolomitization of the carbonatic rock (Warren, 2000). In this contribution, field, petrographic and geochemical studies were carried out in order to determine the temperature and composition of the dolimitizing fluids. The obtained results were then compared to those obtained from Carbon and Oxygen isotopes (Garrido et al., 2001) to discuss the dolimitization process. The mineralized bed, 0.90 m thick, outcrops for about 300 m along the strike (W-E) and 60 m in the dip direction (Fig. 2). This bed pinches out toward the east and toward the west it is no longer visible, it is cover by scours. Petrographic studies determined that the host rock is a dolostone with a breccia texture that becomes more siliciclastic towards the east grading thus to a fine sandstone with carbonatic cement. The hypogenic mineralization, mainly sphalerite, low quantities of galena, pyrite and marcasite is found within the small fractures. Some ghosts of fossils are still visible, but pervasive dolomitization characterizes the horizon. Two distinct dolomites are recognized by crosscutting relationships: a fine to medium grained crystalline dolomite, and a coarse grained crystalline dolomite related to the mineralization. The fomer shows dark-orange and white crystals which occur as patches or partially filled vugs. These crystals are 120-400 mm in size and exhibit subhedral to anhedral shapes (Fig. 3). According to Sibley and Gregg (1987), the texture is no planar -a- unimodal to polimodal. The latter dolomite presents well developed crystals (> 5 mm); they are translucent with pink color and pearl shine and have crystal faces that look like a pavement and is referred as "saddle" dolomite according to Radke and Mathis (1980). This "saddle" dolomite is found into dissolution cavities or as clusters of crystals located on the wall fractures; it is always associated to the mineralization. Chemical analysis of major, traces and rare earth elements are homogeneous throughout the bed. Mean values are 15% MgO, 29,66% CaO and 40,43 % CO2, with high MnO and Fe2O3 contents. The molar percentages of CaCO3 and MgCO3 indicate near stoichiometric ratio (52% and 48%) with a light excess of Ca (Table 1, Fig. 4). The trace elements Sr, Na, Fe and Mn are used to constrain dolomite evolution. Sr values varies from 79 to 159 ppm and Na from 74 to 225 ppm; Mn and Fe contents are higher than the values determined for carbonatic rocks (Turekian and Wedepohl, 1961). ÓREE and LREE contents are low, and the diagram normalized to chondrite shows a negative anomaly of Eu and a great negative anomaly of Ce. The 13C (VPDB) and 18O (VPDB) values vary from -2,9 to -9(0)/00 and from -2,6 to -4 0/00 respectively (Table 2). The 13C are incoherent with the data recorded for Jurassic marine carbonates (near 0 0/00) while 18O values can be correlated with carbonates of the same age (Veizer et al., 1999). Petrography and chemical analysis allow characterizing the depositional environment of the Zn- Pb mineralized dolostone. The xenotopic texture of the dolomite with no planar crystals, gives evidence that the temperatures of deposition should have been higher than 50-60°C (Gregg and Sibley, 1984). On the other hand, the chemical composition, near ideal dolomites (stoichiometric ratio), indicates slow crystallization at high temperature (Morrow, 1982). Morover, the destructive fabric and the homogeneous composition suggest a high temperature dolomitization (Machel, 2004). Trace element values, mainly Na and Sr, agree with burial dolomites, as well as the fluid inclusions reported for these samples by Cesaretti et al. (2002). The negative Ce anomaly indicates that these rocks were formed in a marine environment. Two different processes of carbonate precipitation can produce negative Ce anomaly (Möller, 1989; Bau and Möller, 1992): deposition from seawater or from hydrothermal solutions equilibrated with highly oxydized sediments. The latter is discarded because of the presence of framboidal pyrite and organic matter, which, along with the Eu negative anomaly indicates that the dolimitization were generated under euxinic conditions. This dolostone is in contact with anoxic mudstones (Los Molles Formation, Cuyo Group). Petrographic and geochemical criteria reflect that the dolomitization were caused by normal or modified sea water in a burial environment at temperatures above 140ºC. In burial or altered dolostones, the oxygen isotopes reflect temperature of precipitation and isotope composition of the dolomitizing fluids. The oxygen isotope values of this dolomitized bed are compatible with the isotope composition of carbonates precipitated from sea water at 25°C. The narrow range in the obtained values indicates that there was no influence of meteoric water during this process (Allan and Mathews, 1982). The homogeneous values of 18O isotope suggest that the physic-chemical conditions remained constant during dolomitization, what is in agreement with the textural and geochemical homogeneity found in the study samples. The 18O isotope values of a fluid equilibrated with carbonate at 140°C indicate that the fluid belongs to a basinal fluid. The 13C isotopes reflect an organic origin for the carbon. This carbon came from the diagenesis of organic matter caused by an increase in temperature during the burial of the basin (Garrido et al., 2001; Cesaretti et al., 2002). In contrast with other MVT deposits of the world, in Puesto Gregor, the dolomitization was slow process acting at high temperatures, what has been confirmed by the homogeneity of the fabric and the narrow range in the isotope and trace elements composition. These conditions were reached during burial of the basin where the rocks interact with the basin fluids associated to the ore minerals. <![CDATA[Ambientes de Interacción Eólica-Fluvial en Valles Intermontanos: ejemplos actuales y antiguos]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792008000100004&lng=es&nrm=iso&tlng=es La interacción entre procesos depositacionales eólicos y fluviales en valles intermontanos de regiones áridas y semiáridas determina geoformas y facies sedimentarias particulares, cuyo reconocimiento en sucesiones antiguas puede resultar clave, no sólo desde el punto de vista paleoambiental sino también paleoclimático. Además, los depósitos de interacción pueden mostrar significativa importancia como rocas reservorios. El análisis del valle del río Guandacol (provincia de La Rioja, Argentina) y de tramos específicos de la Formación Vinchina (Sierras Pampeanas Noroccidentales) permitió caracterizar los depósitos de interacción eólica-fluvial tanto en ambientes actuales como en el registro sedimentario. El valle del río Guandacol presenta una planicie entrelazada de canales efímeros asociados a numerosas geoformas eólicas, distinguiéndose dos subambientes: áreas de canal y planicies de intercanal. En las áreas de canal ocurren barras fluviales gravosas, barras y formas de lecho arenosas menores, depósitos arenogravosos de canal propiamente dicho cubiertos parcial o totalmente por láminas de fango; mientras que las geoformas eólicas comprenden parches de arena (por la migración de óndulas de arena y de gránulo), sombras de arena y taludes arenosos. La acción eólica es dominante en las áreas de intercanal, determinando una topografía irregular, de relativamente bajo relieve, por el crecimiento de mantos de óndulas eólicas, formas ancladas a la vegetación (sombras de arena y zibars), protodunas y dunas: barjan y de crestas barjanoides. Se asocian a las geoformas eólicas acumulaciones fluviales formadas durante las crecientes, como mantos de fango y lóbulos de desbordamiento areno-gravosos. Debido al carácter efímero de los cursos fluviales del valle del río Guandacol, puede considerarse que los procesos fluviales y eólicos se encuentran relativamente balanceados lo que resulta en la sobreimposición de depósitos eólicos y fluviales. En tramos del miembro superior de la Formación Vinchina, en facies previamente interpretadas como de fajas de canales meandrosos efímeros, se reconocieron depósitos de interacción eólico-fluviales. Intercalaciones decimétricas en depósitos de canal y de planicie de inundación se identificaron como niveles eólicos. Se caracterizan por la presencia de areniscas finas a muy finas, muy bien seleccionadas y de fábricas depositacionales abiertas, que muestran una delicada laminación, horizontal o entrecruzada de bajo ángulo, con láminas internamente masivas o con gradación inversa de intralámina. El análisis de secciones delgadas muestra como los términos eólicos son mejor seleccionados, presentan muy bajos porcentajes de matriz, y un empaquetamiento más abierto (valores de porosidad de entre 8 y 12%) cuando se los compara con los niveles fluviales. Aunque excepcionalmente citados en el registro geológico, es probable que en ambientes semiáridos los depósitos de interacción eólica-fluvial sean mucho más frecuentes que lo corrientemente asumido. Lo dicho se desprende no sólo de su común presencia en ambientes actuales, sino también del estudio detallado de sucesiones antiguas, como es el caso de la Formación Vinchina.<hr/>Arid to semiarid regions usually show a close interaction between fluvial and aeolian processes resulting in a particular pattern of landforms and sedimentary facies (Langford, 1989; Langford and Chan, 1989). Here we present a characterization of the fluvial-aeolian interaction environment and its distinctive facies, both in modern settings (Guandacol Valley, La Rioja province) and in a sedimentary section of the Vinchina Formation (Northwestern Pampean Ranges). Recognition of these interaction facies in ancient sequences becomes critical not only because of its paleoenvironmental and paleoclimatic significance but also on account of its potential importance as reservoir rocks (Ellis, 1993; Meadows and Beach, 1993). Methodology in Guandacol Valley included mapping of subenvironments by remote sensing and field survey, definition, characterization and sampling of landforms, textural analysis of sediments and description of stratification styles in natural exposures. In the ancient fluvial-aeolian deposits main lithofacies and facies association were identified and described, together with the characterization of sandstones by petrographic studies. Fluvial-aeolian interaction environment in Guandacol Valley (Figs. 1 and 2) is characterized by an ephemeral high-energy gravelly-sandy braidplain associated with abundant aeolian landforms (Tripaldi y Limarino, 1998; Tripaldi, 2002, Tripaldi et al., 2003). This region presents an arid/semiarid regime with average annual precipitations of 130 mm, focus on spring and summer. Two subenvironments have been distinguished in the Guandacol Valley, channel and floodplains (Table 3). The former comprises different kinds of fluvial bars, channel bed deposits and aeolian landforms (Fig. 3). Since most of the year channels remain dry and vegetation cover is scarce, wind action reworked fluvial sediments, determining aeolian rippled mantles and sand shadows (Figs. 4 and 5). According to grain size and morphology two kinds of ripples were recognized in Guandacol Valley: sand aeolian ripples and granule aeolian ripples (Sharp, 1963; Fig. 4). Floodplains are dominated by aeolian landforms (rippled aeolian mantles, sand shadows, zibars, protodunes and dunes; Figs. 6 and 7), with subordinated fluvial deposits (gravelly-sandy overflow mantles and cracked mud drapes; Figs. 6 and 7). Floodplains show an irregular and rolling sandy topography shaped by the emerging of protodunes that evolve to dunes, as well as by the vertical growth of sand shadows and zibars. Although the aeolian sediments could be partially eroded during flood, their importance result from their capacity of producing different types of interactions with fluvial currents. Aeolian bedforms not only can cause temporary dam streams and disruption of the fluvial drainage network (Langford, 1989), but also can supply high quantities of sands promoting rapid saturation of the flooding currents and the consequent amelioration of the flow erosive power. Ancient fluvial-aeolian interaction deposits of the Vinchina Formation (Turner, 1964) are characterized by thin aeolian sandstone bodies interfingered with fluvial rip-up clast conglomerates, sandstones and mudstones deposited in ephemeral meandering plains (Fig. 8). Aeolian levels are 10 to 40-cm thick, tabular to lentiform bodies of well sorted fine to very fine sandstones, showing a very thin parallel or low angle cross-lamination (Fig. 9). Dune deposits were scarcely identified in the studied fluvial-aeolian succession. Remarkable features in the aeolian beds include: 1) inversely graded laminae (product of wind ripple migration), 2) unimodal, symmetrical or positive asymmetric, well to very well sorted sand, 3) open packing and high porosity in sandstones, 4) very low matrix percentage, 5) lack of muddy intraclasts, upper regime structures (as parting lineation) and erosive surfaces, 6) high index ripple forms with coarsest grains at the crest, 7) occurrence of some beds of medium to very coarse (occasionally granule), bimodal sandstones, with parallel to low angle cross-lamination and inversely graded laminae, owing to the development of granule ripples by wind reworking of fluvial sands (Table 4). <![CDATA[Screening of Sediment Pollution in Tributaries from the Southwestern Coast of the Río de la Plata Estuary]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792008000100005&lng=es&nrm=iso&tlng=es Sediment chemistry and textural properties of transported materials from different surface water basins of South America has recently started to be investigated in relation to provenance materials and pollution sources. The objective of the present study is to analyze and compare pollution burdens in bottom sediments from distal positions of three drainage basins running across urban and industrialized areas and to compare them with more preserved sectors (an upstream position and a water stream with low anthropic influence). The surface bodies of water cross wind and water-reworked substrate materials from the Pampean loess and discharge into the Río de la Plata. Sampling was done in distal positions of the Luján and Riachuelo rivers, Canal Oeste, and Juan Blanco creek, and in Las Flores creek, a tributary of the Luján River. Standardized methods for the determination of granulometric parameters, major matrix components, and organic and inorganic pollutants were employed. Assessment of similarities between rivers by Principal Component Analysis show that distal positions of the Luján and Juan Blanco rivers and the tributary group together, and that Riachuelo and Canal Oeste split from that group by the effect of the components 1 and 2. The last two bodies of water also split from each other mainly by effect of component 2. Variables contributing most to the separation of these two bodies of water between each other are mainly given by heavy metals and sulfide. A similar behavior is also shown by cluster analysis.<hr/>Recientemente se han iniciado investigaciones sobre la química de sedimentos y las propiedades texturales de materiales transportados de diferentes cuencas hidrológicas de América del Sur, en relación con la proveniencia de los materiales y fuentes de contaminación. El objetivo del presente estudio es el de analizar y comparar cargas de contaminación de sedimentos de fondo extraídos en posiciones distales de tres cuencas que atraviesan zonas urbanizadas e industrializadas, y compararlos con sectores más preservados (un sector aguas arriba y un arroyo con baja influencia antrópica). Los cuerpos de agua superficial atraviesan sustratos compuestos por materiales retrabajados de origen eólico y fluvial pertenecientes al Loess Pampeano y descargan en el Río de la Plata. Los muestreos fueron realizados en posiciones distales del Río Luján, Riachuelo, Canal Oeste y Arroyo Juan Blanco, además del Arroyo Las Flores, tributario del Río Luján. Se emplearon métodos estandarizados para la determinación de parámetros granulométricos, componentes mayoritarios de la matriz, contaminantes orgánicos e inorgánicos. La evaluación de similitudes entre ríos por Análisis de Componentes Principales indica que las posiciones distales de los ríos Luján y Juan Blanco, además del tributario se agrupan entre sí, y que el Riachuelo y Canal Oeste se separan del grupo por efecto de las componentes 1 y 2. A su vez, estos dos últimos cuerpos de agua se separan entre si por efecto de la componente 2. Las variables que contribuyen en mayor medida a la separación de los mismos entre si, son principalmente los metales pesados y los sulfuros. El análisis de clusters muestra un comportamiento similar al mencionado.