Sample records for DIAGENESIS (diagenesis)
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1

Diagénesis del Cretácico superior manchego

Fernández Calvo, C.

[ES] Con el estudio de los procesos diagenéticos del Cretácico superiormanchego se aportan interesantes datos sobre las característicassedimentológicas y evolutivas de la cuenca de sedimentación. Uncaso especial lo constituye la intensa dolomitización, cuyo análisis adiversas escalas nos ha perm...

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3

Diagénesis y arcillas intersticiales en las unidades neopaleozoicas del Grupo Paganzo-La Rioja/ Diagenesis and intersticial clays of the late Paleozoic Paganzo Group - La Rioja

Andreis, R.R.
2006-09-01

Resumen en español En esta contribución se describen los principales procesos diagenéticos vinculados a las arcillas que ocurren en las areniscas de las unidades Neopaleozoicas del Grupo Paganzo. Se incluyen minerales de la arcillas en las wackes y en los poros de areniscas (caolinita, illita, clorita, y minerales interestratificados), procesos de neoformación (caolinita a dickita) y procesos de cementación (hematita, calcita). Las tobas y tobas pisolíticas con chalazolitas contienen v (mas) itroclastos planares, curviplanares y ramificados y pumicitas vesiculares y están alterados a esmectita y analcima (Formaciones Patquía y La Colina). En la Formación La Antigua, los vitroclastos están alterados en caolinita y analcima. Resumen en inglés In this paper the most important diagenetic clay-related processes that take place in the sandstones from the Neopaleozoic units of the Paganzo Group are described. They include both clay minerals in the wackes and in the pores of clean sandstones (caolinite, illite, chlorite, interestratified clay minerals), processes of neoformation (caolinite to dickite) and of cementation (hematite, calcite). The tuffs and pisolithic tuffs with chalazolites in the Patquía and La Coli (mas) na formations have planar, curviplanar and branched vitroclasts and vesicular pumites and they are also altered into smectite and analcime, while the tuffs of La Antigua Formation, are altered into kaolinite and analcime.

Scientific Electronic Library Online (Spanish)

7

Ambientes de sedimentación, procedencia y diagénesis de depósitos de ríos meandriformes desarrollados sobre playa-lakes. Mioceno de la Cuenca de Loranca (Provincias de Cuenca y Guadalajara)

Arribas, J.; Díaz Molina, M.; Tortosa, A.

[ES] Las sucesiones estratigráficas del Mioceno inferior en la cuenca de Loranca(sistemas deposicionales de Tórtola, Villalba de la Sierra y Valdeganga)presentan depósitos interestratificados de playa-lake y de ríos meandriformes.En este trabajo se estudia un afloramiento donde se encuentra esta ...

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9
11

Carbonatos subtropicales de la Formación Calabozo (Caloviano) y su diagénesis, Cuenca Neuquina, Mendoza/ Subtropical carbonates from the Callovian Calabozo Formation, and their diagenesis, Neuquen Basin, Mendoza province

Palma, R.M.; Mehdli, M.; Bressan, G.S.; Kietzmann, D.A.
2007-06-01

Resumen en español El estudio de los carbonatos calovianos de la Formación Calabozo (30 m) procedente del centro-oeste de Argentina revela la abundancia de partículas carbonáticas bentónicas producidas por organismos fotodependientes y de granos no-esqueletales tales como ooides y peloides. La biota está representada por bivalvos, dasicladáceas (Salpingoporella annulata Carozzi, Cylindroporella annulata Carozzi, y Heteroporella sp.), corales escleractínidos, equinodermos, cianobacter (mas) ias (Cayeuxia (Rivularia) piae Rech-Frollo, y Cayeuxia (Rivularia) kurdistanensis Elliot), oncoides, gastrópodos, foraminíferos, calciesferas y en menor cantidad, briozoos y ostrácodos. Las partículas aloquímicas sugieren para las calizas marinas someras de la Formación Calabozo una asociación Photozoan de aguas subtropicales. Las características petrográficas permitieron reconocer siete microfacies diferenciadas de acuerdo al tipo de partículas, fósiles y textura depositacional: 1) bindstone criptalgal; 2) wackestone bioclástico-peloidal; 3) wackestone bioclástico-intraclástico; 4) wackestone bioclástico-peloidal-terrígeno, 5) boundstone coralino, 6) packstone bioclástico-peloidal-intraclástico, y 7) packstone/grainstone oolítico-bioclástico-peloidal. El reconocimiento de grupos de microfacies constituyen dos asociaciones paleoambientales: rampa interna y rampa media. Los procesos de micritización y precipitación de micrita peloidal y cemento acicular son sinsedimentarios. Estos cementos son previos a la fase de cementación de calcita granular y sintaxial. Estudios de cátodoluminiscencia permitieron identificar tres tipos de cementos: (1) acicular, (2) calcita granular, y (3) cemento sintaxial en partículas de equinodermos. La cátodoluminiscencia permitió determinar cementos no luminiscente, cemento con luminiscencia opaca (roja/naranja) y cemento con luminiscencia amarilla brillante. El cemento acicular es no luminiscente (marino), en tanto que la luminiscencia opaca a brillante típica de las cavidades inter-intraparticulares y del cemento sintaxial indica un ambiente diagenético meteórico-freático. Resumen en inglés Detailed study of Callovian carbonates of the Calabozo Formation (30 m) in western Argentina, reveals the abundance of benthic carbonates particles produced by organisms that are light dependent, and non-skeletal grains such as ooids and peloids. The biota consists of bivalves, dasyclads (Salpingoporella annulata Carozzi, Cylindroporella annulata Carozzi, and Heteroporella sp.), scleractinian corals, echinoderms, cyanobacteria (Cayeuxia (Rivularia) piae Rech-Frollo, and C (mas) ayeuxia (Rivularia) kurdistanensis Elliot), oncoids, gastropods, forams, and calcispheres as well as ostracods, and bryozoans in relatively minor quantities. Allochemical particles suggest for the shallow marine limestones of the Calabozo Formation a Photozoan Association under subtropical seawater conditions. The petrographic features allow to recognize seven microfacies, differentiated by particle types, fossils and depositional texture: 1) criptalgal bindstone; 2) bioclastic-peloidal wackestone; 3) bioclastic-intraclastic wackestone; 4) bioclastic-peloidal-terrigenous wackestone, 5) coral framestone, 6) peloidal-bioclastic-intraclastic packstone, and 7) oolitic-bioclastic-peloidal packstone-grainstone. The recognition of microfacies groups constitute two palaeoenvironmental associations: inner ramp and middle ramp settings. Diagenetic studies are based on examination of stained thin sections, through conventional petrography and under cathodoluminescence. Early diagenetic processes are micritization and precipitation of peloidal micrite and acicular calcite. These cements predate granular and syntaxial calcite cements. Cathodoluminescent petrography allows to identify three morphologic types of clear cements: (1) fine acicular crystals, (2) granular cements, and (3) syntaxial cement rims on echinoderm particles. Description of the CL signature has evolved to discrimination as either, non-luminescent, dully luminescent red/orange, or brightly luminescent yellow. The earlier acicular calcite cement is non-luminescent, on the other hand, the predominantly dully luminescent red/orange, or brightly luminescent yellow of interparticular and intraparticular granular calcite and syntaxial cements indicate a shallow burial meteoric-phreatic diagenetic environment.

Scientific Electronic Library Online (Spanish)

12

Contribución al estudio de la diagénesis de los carbonatos sedimentarios

Gómez de Llarena, J.

[ES] En la formación de los carbonatos marinos magnesitay dolomita, nacidos al borde del mar en lagunas, limanes o albuferas,intervienen factores ciimáticos cuya importancia era hasta ahora apenas conocida El proceso genético es, sin embargo,largo y aun desconocido en su mayor parte. El término"...

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15

Condiciones paleoambientales de sedimentación y preservación de materia orgánica en bahía Mejillones del Sur (23º S), Chile/ Paleo-environmental sedimentation conditions and organic matter preservation in Mejillones del Sur Bay (23º S), Chile

Valdés, Jorge; López, Liliana; LoMónaco, Salvador; Ortlieb, Luc
2000-12-01

Resumen en español El estudio combinado de metales y de materia orgánica soluble contenidos en un testigo de sedimentos de 48 cm de longitud ha permitido identificar las condiciones paleoambientales de sedimentación y preservación de materia orgánica en la bahía Mejillones del Sur (23ºS, norte de Chile). Concentraciones altas de metales como Cd y Mo, ambos indicadores de paleoxigenación, y el uso de los índices redox V/Cr y Ni/Co a lo largo del testigo evidenciaron el predominio de (mas) condiciones de depositación disóxicas/anóxicas en el fondo de la bahía durante el último milenio. El análisis IRTF realizado sobre la materia orgánica soluble y su fracción de hidrocarburos saturados evidenció que en el transcurso de los pasados mil años la diagénesis temprana ha afectado en grado mínimo a los constituyentes orgánicos del sedimento, ya que los espectros obtenidos mostraron que los grupos funcionales presentes en ellos son los mismos que normalmente pueden encontrarse en los precursores biológicos (organismos vivos). El alto contenido de materia orgánica y su buen estado de preservación tienen relación directa con la pobreza de oxígeno que han presentado las aguas subsuperficiales de la bahía, durante el último milenio Resumen en inglés A combined study bearing on the downcore variations of metal elements and of organic matter composition in a 48 cm-long sedimentary core provided elements to reconstruct paleoenvironmental conditions of oxygen availability and organic matter preservation in the Bay of Mejillones del Sur (23°S, northern Chile). High concentration of heavy metals like Cd and Mo, both indicators of former conditions of oxygenation, and the values of redox indices V/Cr y Ni/Co in several poi (mas) nts of the core, suggest a predominance of disoxic/anoxic conditions at the bottom of the water column all along the last millenium. The transmission Fourier transformed infrared (FTIR) spectroscopy of the soluble organic matter showed that, during the last thousand years, the early diagenesis scarcely affected the organic components of the sediment: the spectra indicate that the functional groups are the same that those of the biological precursors (living organisms). The high content of organic matter and its good state of preservation reflect the general lack of oxygen experienced by the subsuperficial waters of the embayment during the last millenium

Scientific Electronic Library Online (Spanish)

16

Sedimentología, cicloestratigrafía y análisis secuencial de la Formación La Manga (Oxfordiano), Bardas Blancas, Mendoza/ Sedimentology, ciclostratigraphy and sequence analysis of the La Manga Formation (Osfordian), Bardas Blancas, Mendoza

Piethé, Ricardo D.; Palma, Ricardo M.
2008-12-01

Resumen en español El estudio de los depósitos de la rampa carbonática de la Formación La Manga (60 m) ha permitido reconocer tres asociaciones de facies. La asociación de facies A está compuesta por cinco litofacies (A1-A5). Esta asociación representa parte del cortejo transgresivo que aparece representado por ciclos de espesores centimétricos-decimétricos y pertenecerían a parasecuencias retrogradantes de 5º orden. La asociación de facies B se inicia, sobre la asociación de fa (mas) cies A, luego de un abrupto contacto de facies y está caracterizada por seis litofacies (B1-B6). Se interpreta como depósitos de rampa media dominados por tormentas. Corresponderían a parasecuencias de 4º orden. La asociación de facies C está compuesta por tres litofacies (C1-C3). Los ciclos de pequeña escala, reconocidos en la Asociación de facies A y B, estuvieron controlados por eventos de tormenta, en tanto que aquellos de mediana y gran escala, también reconocidos en las asociaciones de facies A, B y C, fueron originados probablemente por eustasia. Las secciones estratigráficas estudiadas de la Formación La Manga se incluyen dentro de dos secuencias depositacionales SD-2 y SD-3. Una abrupta discontinuidad marca el inicio de la secuencia depositacional SD-2 que constituye el cortejo transgresivo compuesto por un apilamiento de parasecuencias somerizantes hacia el techo. A partir de la superficie de máxima inundación (SMI), reflejada por un cambio drástico de facies, se evidencia un período de disminución del espacio de acomodación y caída del nivel del mar dado por un cortejo de mar bajo (CMB). La tercera secuencia depositacional (SD-3) está constituida por los depósitos de albufera y se inicia luego de una superficie paleocárstica con el desarrollo de una brecha mantiforme y una intensa diagénesis vadosa. Resumen en inglés The study of the carbonate-ramp deposits of La Manga Formation has allowed us to recognize three facies associations. The lithofacies association A is composed by five lithofacies (A1-A5). This association represents part of the transgressive systems tract. It corresponds to external ramp deposits, accumulated below the storm-wave base. The A facies association appears in cycles of centimetric to decimetric thickness and belong to a retrograding parasequence set of 5th or (mas) der. The lithofacies B association starts after an abrupt facies contact. It is characterized by six lithofacies (B1-B6). They are coarsening upward beds that form metric-scale cycles. These are interpreted as middle-ramp deposits dominated by storms. It corresponds to 4th order sequences. The lithofacies C includes three lithofacies (C1-C3). The small scale cycles were controlled by storm events, while the medium and large scale ones were likely originated by eustasy. The studied stratigraphic sections of La Manga Formation are included in two depostitional sequences SD-2 and SD-3. An abrupt discontinuity marks the beginning of the depositional sequence SD-2, which constitutes a transgressive system-tract, composed by a coarsening upward parasequence stacking set. Above a maximum flooding surface, manifested by a drastic change of facies, a low rate accommodation space period and a sea-level fall (characterized by a lowstand systems tract) are evidenced. The third depositional sequence SD-3 is composed by lagoon deposits. It starts after a paleokarstic surface, characterized by the development of a sheet-cake breccia and an intense vadose diagenesis.

Scientific Electronic Library Online (Spanish)

17

Uso de las Propiedades Fisicoquímicas de Oligonucleótidos como Biomarcadores/ Use of oligonucleotid physicochemical properties as biomarkers

Martínez Reyes, José; González Partida, Eduardo; Perez, Renee J.
2009-01-01

Resumen en español Marcadores biológicos (biomarcadores) son complejos moleculares que pueden estar presentes en fósiles, procedentes de biomoléculas de los organismos vivos. Debido a sus características generales son resistentes a la intemperie, la biodegradación, la evaporación y otros procesos biológicos, siendo comúnmente conservados en las rocas y pueden ser utilizados por geólogos, geoquímicos, arqueólogos, etc. para obtener información sobre la materia orgánica en las ro (mas) cas fuente, la presencia de petróleo, las condiciones ambientales durante su sedimentación (diagénesis), la madurez térmica experimentada por el petróleo y/o roca (catagénesis), el grado de biodegradación, algunos aspectos de la mineralogía de la roca fuente (litología), la edad de los fósiles y en caracterización de DNA (Acido Desoxirribonucleico). Los biomarcadores pueden ser oligonucleótidos, que son fragmentos de moléculas de DNA de cadena sencilla de determinada longitud. El DNA de fósiles puede formar enlaces cruzados entre si o con otras moléculas al paso del tiempo, dificultando el uso de técnicas como el PCR para su estudio, por ello, en el presente trabajo se aplicó a los oligonucleótidos la técnica de determinación de los valores de pKa (potencial de la constante de ionización), mediante espectrofotometría de luz ultravioleta en el intervalo básico de pH, usando como oligonucleótido modelo el d-AAAGAAA en solución acuosa de NaCl, titulándolo con solución de NaOH. Lo anterior también se realizo en presencia de una solución amortiguadora de pH (NaHCO3). Además se determinaron valores de pKa para el mismo compuesto a diferentes condiciones de temperatura, sales y mezclas de alcohol-agua como solvente. También se determinaron valores de pKa a los oligonucleótidos d-CCCGCCC y d-AAGAA. Los valores de pKa obtenidos varían entre 9 y 12. Estos resultados obtenidos son un aporte al conocimiento de las propiedades físicas de los oligonucleótidos, estos datos pueden ser especialmente útiles en el mencionado caso de dificultad para el uso de otras técnicas como el PCR para la caracterización e interpretación de los oligonucleótidos como biomarcadores. Resumen en inglés Biological markers (biomarkers) are complex molecular fossils from biomolecules in living organisms. Due to their general are resistant to weather, the biodegradation, evaporation and other biological processes, as commonly preserved in rocks and can be used by geologists, geochemists, archaeologists, etc. for information on organic matter in source rocks, the presence of oil, environmental conditions during sedimentation (diagenetic process), the thermal maturity experie (mas) nced by the oil and / or rock (catagenetic process), the degree of biodegradation, some aspects of mineralogy of the source rock (lithology), the age of fossils and characterization of DNA (deoxyribonucleic acid). Biomarkers can be oligonucleotides, which are stretches of DNA molecules of simple fixed-length string. DNA from fossils can form cross-links among themselves or with other molecules to the passage of time, hindering the use of techniques such as PCR for study, therefore, in this work was applied to oligonucleotides a technique for determining the pKa values (potential of the ionization constant), by UVspectrophotometry in the basic pH range, using as the model oligonucleotide d-AAAGAAA in NaCl aqueous solution, titrating it with NaOH solution. Also in the presence of a pH buffer solution (NaHCO3). Furthermore pKa values were determined for the same compound at different conditions of temperature, salts and mixtures of alcohol-water as solvent. We also assessed the pKa oligonucleotides d-CCCGCCC and d-AAGAA. pKa values were obtained on a range between 9 and 12. These results are a contribution to the knowledge of the physical properties of oligonucleotides, such data may be particularly useful in the aforementioned case of difficulty using other techniques such as PCR for the characterization and interpretation of oligonucleotides as biomarkers.

Scientific Electronic Library Online (Spanish)

18

Petrográfica y Geoquímica de la dolomía hospedante de una mineralización de Zn y Pb. Puesto Gregor, Neuquén, Argentina/ Petrography and geochemistry of a Zn- Pb mineralization hosted in a dolostone, Puesto Gregor, Neuquen, Argentina

Gómez, M. Cristina; Garrido, Mirta; Cesaretti, Nora; Domínguez, Eduardo
2008-07-01

Resumen en español 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 (mas) 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. Resumen en inglés 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 (mas) 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.

Scientific Electronic Library Online (Spanish)

19

Use of laser ablation ICP-MS to determine trace element distributions in coals, with special reference to V, Ge and Al

Spears, D. A.; Gómez Borrego, Ángeles; Cox, A.; Martínez Tarazona, M.ª Rosa
2007-02-20

Digital.CSIC (Spain)

23

The hot spring and geyser sinters of El Tatio, Northern Chile

Fernandez-Turiel, J. L.; García-Vallés, M.; Gimeno, D.; Saavedra Alonso, Julio; Martínez-Manent, S.
2005-10-15

Digital.CSIC (Spain)

24

The diagenesis in the anticline of Bilbao, Vasco Cantabrian basin (Spain)

Aróstegui, J.; Velasco, F.; Ortega-Huertas, M.
1987-01-01

Digital.CSIC (Spain)

26

The Río Tinto Basin, Spain: Mineralogy, sedimentary geobiology, and implications for interpretation of outcrop rocks at Meridiani Planum, Mars

Fernández-Remolar, David; Morris, Richard V.; Gruener, John E.; Amils, Ricardo; Knoll, Andrew H.
2005-11-02

Digital.CSIC (Spain)

27

Structural characterization of kerogen in 3.4 Ga Archaean cherts from the Pilbara Craton, Western Australia

Marshall, Craig P.; Love, Gordon D.; Snape, Colin E.; Hill, Andrew C.; Allwood, Abigail C.; Walter, Malcolm R.; Van Kranendonk, Martin J.; Bowden, Stephen A.; Sylva, Sean P.; Summons, Roger E.
2007-05-05

Digital.CSIC (Spain)

28

Stable Eocene Magnetization Carried by Magnetite and Iron Sulphides in Marine Marls (Pamplona-Arguis Formation, Southern Pyrenees, Northern Spain)

Larrasoaña, Juan C.; Parés, Josep M.; Pueyo, Emilio L.

In order to establish the magnetic carriers and assess the reliability of previous paleomagnetic results obtained for Eocene marine marls from the south Pyrenean basin, we carried out a combined paleo- and rock-magnetic study of the Pamplona-Arguis Formation, which crops out in the western sector of...

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Sandstone petrofacies in the northwestern sector of the Iberian Basin

Arribas, J.; Ochoa, M.; Mas, R.; Arribas, Mª E.; González-Acebrón, L.
2007-01-01

Digital.CSIC (Spain)

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Retrograde diagenesis, a widespread process on a regional scale

Nieto, F.; Mata, M. Pilar; Bauluz, B.; Giorgetti, G.; Árkai, P.; Peacor, D. R.
2005-03-01

Digital.CSIC (Spain)

36

Procesos diagenéticos en las rocas carbonáticas del Muschelkalk de la Cordillera Ibérica

Peña, J. A. de la; Yébenes, A.

[ES] Se ha realizado el estudio de más de 500 muestras, correspondientes a17 columnas, con el objeto de analizar detalladamente las transformacionespostsedimentarias y sus variaciones en el tiempo y en el espacio.Fundamentalmente se ha tratado de esclarecer la situación temporal de ladolomitizac...

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Procesos diagenéticos en las areniscas del Buntsandstein de la Cordillera Ibérica

Peña, J. A. de la; Cruz, B. de la; Marfil, R.

[ES] Composicionalmente los materiales estudiados son, en general, arcosas ysubarcosas, con cantidades variables de matriz arcillosa, cementadas por dolomita,calcita, sílice (opalina o de crecimiento sintaxial sobre los granos decuarzo), óxidos de hierro y, esporádicamente, baritina.Se abordan l...

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Perfiles paleokársticos en el techo de la unidad intermedia del mioceno de la cuenca de Madrid

Cañaveras, J. C.; Sánchez-Moral, S.; Ordóñez, S.; Calvo Sorando, J. P.

An intra-Vallesian (Upper Miocene) paleokarst developed at the top of the Intermediate Miocene Unit in the continental Madrid basin is recognized. This paleo karst is an early shallow, tabular-shaped karst that shows a marked control by the depositional facies pattern and lithologies. By integratin...

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Perfiles paleokársticos en el techo de la unidad intermedia del mioceno de la cuenca de Madrid

Cañaveras, J. C.; Sánchez-Moral, S.; Ordóñez, S.; Calvo Sorando, J. P.
2003-08-30

Digital.CSIC (Spain)

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Organic matter accumulation during the Holocene in the Guadalquivir marshlands (SW Spain)

Ortiz, José E.; Torres, Trinidad; López-Pamo, José E.; Soler, V.; LLamas, Juan F.; Barettino, Daniel; García, María J.
2007-07-17

Digital.CSIC (Spain)

46

Models of iron oxide concretion formation: field, numerical, and laboratory comparisons

Chan, M. A.; Ormö, Jens; Park, A. J.; Stich, Michael; Souza-Egipsy, Virginia; Komatsu, G.
2007-06-06

Digital.CSIC (Spain)

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53

La edad de las ilitas en areniscas pérmicas y triásicas de la Cordillera Ibérica mediante el método K/Ar: Implicaciones en la historia diagenética y evolución de la cuenca.

Penha Do Santos, R.; Bonhomme, M. G.; Peña, J. A. de la; Sell, I.

[ES] Las areniscas pérmicas y triásicas de un sondeo de casi mil metros,localizado en Siglienza (SS-44-3), Cordillera Ibérica, con una composiciónde sublitoarenitas-grauvacas y arcosas-subarcosas, respectivamente,tienen cementos de ilita formados en varias etapas de su historia diagenética,como ...

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Isotopic signatures of extinct low-temperature hydrothermal chimneys in the Jaroso Mars analog

Martínez-Frías, Jesús; Delgado Huertas, Antonio; García-Moreno, Francisco; Reyes, Emilio; Lunar, Rosario; Rull, Fernando
2007-03-01

Digital.CSIC (Spain)

57

In situ trace element analysis of macerals and minerals in coals using laser ablation ICP-MS

Spears, D. A.; Gómez Borrego, Ángeles; Cox, A.; Martínez Tarazona, M.ª Rosa
2007-01-01

Digital.CSIC (Spain)

58

Illitic Substitution in Micas of Very-Low-Grade metamorphic Clastic Rocks.

Abad, Isabel; Nieto, F.; Gutiérrez-Alonso, G.; Campo, Margarita do; López-Munguira, Aurora; Velilla, Nicolás
2006-02-01

Digital.CSIC (Spain)

59

Geoquímica orgànica de conques lacustres fòssils

Grimalt, Joan O.; Heras i Cisa, Xavier de las
2009-01-01

Digital.CSIC (Spain)

63

Evolución diagenética de los carbonatos arrecifales de la Formación Torrecilla en Cameros y de los carbonatos continentales suprayacentes (Kimmeridgiense inferior-Titónico) en el Sector de Soria. Cuenca de Cameros, N. España | Diagenetic evolution of the reefal carbonates of the Torrecilla en Cameros Formation and overlying continental carbonates (Early Kimmeridgian Tithonian) in the Soria Sector. Cameros Basin, N. Spain

Benito, M. I.; Mas, R.

[ES] El último episodio de sedimentación marina del Jurásico en el área de Soria se produjo durante el Kimmeridgiense inferior con el depósito de la Fm. Torrecilla en Cameros, formada por areniscas y calizas arrecifales y oolíticas que se depositaron en una rampa carbonática somera. Fue durante el d...

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Estratigrafía del Grupo Mina Verdun, Proterozoico de Minas, Uruguay/ Stratigraphy of the Mina Verdún Group, Proterozoic of Minas, Uruguay

Poiré, Daniel G.; González, Pablo D.; Canalicchio, José M.; García Repetto, Flavio
2005-12-01

Resumen en español El objetivo del presente trabajo es el de presentar un cuadro estratigráfico para las sedimentitas del área del Cerro Verdún, Minas, Uruguay. Asimismo presentar algunas consideraciones sobre sus características sedimentológicas y sus estromatolitos, a fin de discutir sus condiciones paleoambientales. En el mismo sentido, se realizan inferencias sobre la edad abarcada por estas rocas y sus potenciales correlaciones regionales. Esta sucesión ha permanecido indefinida (mas) en general, mientras que sus calizas fueron alternativamente asignadas a diversas unidades litoestratigráficas. En esta contribución se incluye al conjunto en el Grupo Mina Verdún, el cual está conformado de base a techo por: i) Formación Don Mario (40 m, base no expuesta, lutitas negras), ii) Formación La Toma (15 m, margas verdes oscuras a negras), iii) Formación El Calabozo (170 m, calizas grises, masivas, laminadas y estromatolíticas) y iv) Formación Gibraltar (60 m, techo no expuesto, dolomías en tonalidades pálidas amarillentas, verdes y rosas hasta gris oscuras más calizas rosas y pelitas y margas negras). Conglomerados polimícticos de la Formación Las Palmas cubren a esta unidad mediante una discordancia angular. En contraposición, las cuarcitas del Cerro Verdún se apoyan sobre el Grupo Mina Verdún por contacto tectónico. Las pelitas negras de la Formación Don Mario se habrían depositado en un ambiente reductor de aguas calmas. Se identifica para las mismas, de manera preliminar, un aporte desde una corteza continental superior diferenciada que sufrió una importante meteorización y retrabajo. Las margas de la Formación La Toma reflejan un cambio en la sedimentación, con un pasaje hacia un ambiente de rampa carbonática. Niveles con estromatolitos aparecen en las calizas de la Formación El Calabozo, constituidos esencialmente por columnas de laminación cónica asignables a Conophyton fm. y muy escasos estromatolitos columnares dendroides, mal preservados. Conophyton es considerado como un estromatolito submareal de aguas relativamente profundas. Esto último, sumado a la baja diversidad, sugiere un ambiente marino submareal externo para esta unidad. Correlaciones entre afloramientos locales, definirían una evolución desde una rampa carbonática hasta una plataforma, en las inmediaciones del talud de la misma. La edad de esta unidad aún no se conoce con precisión. Sin embargo, interpretaciones geológicas regionales, sobre la base de los estromatolitos presentes y de datos geoquímicos, sugieren un tiempo de depositación post-arqueano a pre-ediacareano. Resumen en inglés The aim of this contribution is to introduce the stratigraphic framework of the Mina Verdún Quarry, located in the eastern side of Verdún Hill, Minas, Uruguay (Fig. 1), and to characterise their sedimentary units. The paleoenvironments involved are outlined as well as the sedimentological significance of the very well developed stromatolites. The stratigraphic position and composition of this sedimentary succession used to be uncertain and the limestone from the Mina Ve (mas) rdún Quarry was alternatively considered part of diverse litostratigraphic units (Mac Millan, 1933; Caorsi and Goñi, 1958; Bossi et al., 1965; Midot, 1984; Preciozzi et al., 1985; Bossi and Navarro, 2000; Sánchez Betucci, 1998; Sánchez Betucci et al., 2001). From the base to the top (Fig. 2), the sedimentary succession consists of the Mina Verdún Group, Las Palmas Formation and the quartzites of the Verdún Hill (Fig. 3). Mina Verdún Group (Poiré et al., 2003b) is conformed by: Don Mario Formation (Poiré et al., 2003b): 40 m thick, base not exposed; massive and laminated, black shales (Fig 4a), which are rich in organic matter (Fig. 5a). XRD analysis shows manly quartz, with scarce calcite, clay (illite, chlorite, and IS and CS interlayer) and feldspars (Fig. 6). Applying the Bhatia and Crook (1986), Taylor and McLennan (1985) and McLennan et al. (1990, 1993) plots (Fig. 7) their geochemistry suggests a high degree of weathering and reworking in the source area compose of a differentiated upper continental crust. La Toma Formation (Poiré et al., 2003b): 15 m thick; fine-grained, dark green marls (Fig. 4b and c), bearing up to 45 % of silica and 35 % of CaCO3. Petrographically they show micritic crystals of calcite and dolomite, quartz and feldspars (Fig. 5b); lamination is interrupted by stilolytes. Clay minerals are composed mainly by illite and scarce chlorite (Fig. 6). El Calabozo Formation (Poiré et al., 2003b): 170 m thick; grey massive, laminated, and stromatolite limestones (Fig. 4d), bearing up to 93 % of CaCO3 with some collapse breccias as resulting of probably karstic phenomena. In thin section they show homogeneous mosaic of micrite and subsparite calcite (Fig. 5 c and d) with stilolites and corrugated stromatolite lamination. Very abundant calcite with rare quartz, feldspars and illite have been recorded by XRD (Fig. 6). This sedimentary sequence is affected by diagenetic features and a fragile deformation (cleavage foliation, shear zones) which are slightly hiding the stromatolite morphologies (Fig. 8), but the main features of the stromatolites from El Calabozo Formation are still well preserved. Columnar stromatolites assignable to Conophyton fm. are the most abundant group of these organic sedimentary structures. This Conophyton fm. consists of unbranching subcylindrical columnar stromatolites with strikingly conical internal laminae whose apexes define a distinctive axial zone (Figs. 10a, b, c) and their horizontal section display a conspicuously and regularly concentric structure (Figs. 10c, d, f). The internal laminae are commonly continuous from one column to another, in which case vertical sections show upward laminae apexing. Their profile is angulate to geniculate and their plant outline could be round circular to oblong, in which case the axes are strongly orientated. The attitude of the columns is usually straight but sometimes they adopt a recumbent and sinuous posture. The column height is up to 70 cm and the column width is 5 to 20 cm. However, scarce but preserved dendroid branching style stromatolites have been recorded on the field, as well as small, rounded plant outline columnar stromatolites have been observed in cores, which could represent digitate branching stromatolites. Five kilometres to the south, this stromatolitic limestone facies change to deformed laminate limestones (Figs. 4e, f). Furthermore, 40 km to south, the El Calabozo Formation bearing Conophyton stromatolites has been reported by Gaucher et al. (2004a). Gibraltar Formation (Poiré et al., 2003b): 60 m, top not exposed; black marls and shales (Figs, 10a, b, c), pink limestones and dolomite limestones, and light yellow, green, reddish, black and grey dolomites (Fig. 10d, e). The unconformity between the El Calabozo and Gibraltar formations is very irregular, which is interpreted as a karstic palaeorelief. A 4 m thick diagenetic breccia is often in the base of the unit. The Mina Verdún Group is overlaid by red polymictic conglomerates and sandstones (Fig. 10f) of Las Palmas Formation (Poiré et al., 2003b) and is intruded by a subvolcanic longitudinal (E-W) dike swarm. This shows a K-Ar whole rock datum of 485,2 ± 12,5 Ma considered as a minimum cooling (Gonzalez et al., 2004). The quarzites from the Verdún Hill are overlying these units by a tectonic contact. They have been assigned to the Cerro Espuelitas Formation by Gaucher (2000), which are Ediacarean in age. Sedimentological and stratigraphic implications The black shales and claystones of the Don Mario Formation are interpreted as fine deposits in a low energy environment with euxinic conditions, which becomes calcareous to form a carbonatic ramp (La Toma Formation). The latter shows and evolution to a carbonatic algae platform very rich in stromatolites. From a sedimentological point of view, the group Conophyton has been assigned as a deep subtidal stromatolite by Poiré (1987, 2002) based on Precambrian stromatolite cycles from Villa Mónica Formation, Buenos Aires Province, Argentina, and Logan et al. (1964) and Donaldson (1976) ideas. In this sense, the low biodiversity, Conophyton abundance and the plant view axes orientation allow to suggest a subtidal marine environment for the El Calabozo Formation, with tidal currents influence (Fig. 11). The strong dolomitization of the Gibraltar Formation and their poor exposure do not allow doing detailed interpretation about paleoenvironments. The age of the Mina Verdún Group is Precambrian but there are not major precisions about that. Unfortunately, the group Conophyton has a wide range during the Precambrian, from Early Proterozoic to Ediacarean, being impossible to define the age of these limestones. More detailed future studies about microstructure of this Conophyton could be useful to distinguish the taxonomy and to determinate its possible age. Pre-Ediacarean stromatolites have also been described 30 km to the north by Sprechmann et al. (1994) and Gaucher et al. (1996) in the Villalba Formation of the Basal Group (Gaucher y Sprechmann, 1995), but their morphologies are completely different. They have recorded stratiform, nodular-stratiform (LLH-C, LLHV) and columnar (SH-V) stromatolites associated with stromatolitic breccias, which were interpreted as intertidal to supratidal deposits. It could represent shallower stromatolite assemblages than the stromatolites from the Mina Verdún. Unfortunately, there are no radiometric data to prove this idea. Very well developed Conophyton ?ressotti and Conophyton fm. have been also recorded in the dolomites of the Villa Mónica Formation (Poiré, 1993) companied by Colonella fm., Cryptozoon fm., Gongylina fm., Gymnosolem fm., Inzeria fm., Jacutophyton fm., Jurusonia nisvensis, Katavia fm., Kotuikania fm., Kussiella fm., Minjaria fm., Parmites fm., Parmites Cf. cocrescens and Stratifera fm. (Poiré, 1989 y 1993), which radiometric age for diagenesis is 795 My (Cingolani y Bonhomme, 1988). This sequence could be correlated with El Calabozo Formation but it shows a poor biodiversity. The Mina Verdún Group is considered as a new lithostratigraphic unit in the Uruguayan stratigraphic framework, which is probably located somewhere between the Basal and Arroyo del Soldado groups.

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El yacimiento de Las Higueruelas, Alcolea de Calatrava (Ciudad Real): procesos diagenéticos y volcanismo asociado | The paleontological site of Las Higueruelas, Alcolea de Calatrava (Ciudad Real): Diagenetic processes and associated volcanism

Rodríguez Badiola, E.; Mazo, A. V.; Rodríguez Ruiz, P.

El yacimiento de Las Higueruelas constituye un escenario singular para el estudio de procesos diagenéticos relacionados con un contexto tafonómico complejo. Existen en él numerosos fósiles de vertebrados e invertebrados del Plioceno, preservados en unas particulares condiciones de enterramiento,lagu...

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El protolito siliciclástico de las Ectinitas El Jagüelito, provincia de Río Negro/ The siliclastic protolith of the El Jagüelito ectinites Río Negro provimce

Dalla Salda, LH; Aragón, E; Benialgo, A; Abre, P; Pezzotti, C
2003-09-01

Resumen en español Mediante el análisis del protolito se determinó el carácter de los sedimentos madre de las Ectinitas El Jagüelito, localizadas en la secuencia de metamorfitas del basamento cristalino del este del Macizo Norpatagónico. Se realizaron estudios que incluyeron petrografía, análisis con conteo de granos, relaciones y composición del bandeado y rayos X. Las rocas investigadas derivaron de grauvacas y arenitas líticas, principalmente de grano psamítico y pelítico, dep (mas) ositadas en una cuenca marina profunda por corrientes de alta densidad, provenientes de un arco magmático cercano. Escasas rocas ricas en cuarzo y otras de bajo contenido en carbonato de calcio, pueden representar ambientes más neríticos. El sistema químico de las muestras analizadas no parece haberse modificado por alteración, transporte o metamorfismo. El comportamiento de muchos elementos relevantes en el estudio de protolitos, sugiere que el metamorfismo fue isoquímico. Los diagramas de discriminación de ambientes tectónicos, apuntan a un margen activo como fuente de procedencia de los sedimentos. Recientes datos geocronológicos parecen indicar que el arco continental y la sedimentación del protolito, ocurrieron durante el Cámbrico. Resumen en inglés The sedimentary character of the El Jagüelito ectinites, within the classic sequence of the crystalline basement of eastern North Patagonian Massif has been established by protolith analysis. A range of studies from hand specimen to trace elements analysis and including micro section, single grain, banding relations and X-ray analysis were performed. The rocks are derived from pre-existing greywacke and lithic sandstone, of shale to sandy grain size. They were deposited (mas) within a deep-sea basin, as high-density current deposits coming from a magmatic arc. Sparse quartz- rich rocks and calcium-poor carbonate rocks may represent a more neritic environment. Weathering, transportation and diagenesis seem not to modify the HFS elements. The behaviour of chemical elements, which are relevant to protoliths, suggests that metamorphism was isochemical. Discriminate tectonic diagrams indicate an active margin as the source of the sediments. Recent data from the region seem to indicate a Cambrian age for the continental arc and protolith sedimentation.

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Diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems.

Larrasoaña, Juan C.; Martínez-Ruiz, Francisca; Vega, Marta; Piñero, Elena; Gràcia, Eulàlia; Musgrave, Robert J.; Roberts, Andrew P.
2007-09-30

Digital.CSIC (Spain)

72

Diagenesis of the Central Basque-Cantabrian Basin (Iberian Peninsula) based on Illite-Smectite Distribution

Aróstegui, J.; Zuluaga, M. C.; Velasco, F.; Ortega-Huertas, M.; Nieto, F.
1991-11-01

Digital.CSIC (Spain)

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Caracterización de los lignitos garumnienses del Pirineo Central catalán.

García-Vallés, M.; González-Prado, J.; Vendrell-Sanz, M.
1993-01-01

Digital.CSIC (Spain)

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Bacterially-Mediated Authigenesis of clays in Phosphate Stromatolites

Sánchez-Navas, A.; Martín Algarra, A.; Nieto, F.
1998-05-01

Digital.CSIC (Spain)

86

Análisis de facies y paleoambiental de alta resolución de la Aloformación Punta San Andrés (Plio-Pleistoceno), provincia de Buenos Aires, Argentina

Beilinson, Elisa
2009-07-01

Resumen en español La Aloformación Punta San Andrés aflora principalmente a lo largo de las barrancas costeras que se desarrollan desde la ciudad de Mar del Plata hasta la ciudad de Miramar y comprende las sedimentitas de edad plio-pleistocenas interpretadas como depósitos continentales de planicies aluviales, sistemas fluviales de baja sinuosidad y cuerpos de agua someros. El presente trabajo tiene por finalidad efectuar un análisis de alta resolución de facies sedimentarias así como (mas) de asociaciones de facies y arquitectura de los cuerpos sedimentarios presentes en los afloramientos de Baliza San Andrés y Complejo Turístico Chapadmalal de la Aloformación Punta San Andrés con el propósito de elaborar un modelo conceptual y dinámico de los ambientes depositacionales que la conformaron. Basado en el trabajo de campo y de laboratorio realizado, se definieron en primera instancia dos grandes grupos de facies, unas de origen depositacional y otras de origen postdepositacional, las cuales se agruparon en cuatro asociaciones de facies según los procesos que le dieron origen y según su arquitectura (AF I a IV: canales fijos de baja sinuosidad, canales móviles de alta sinuosidad, lóbulo de explayamiento y planicie de inundación pedogenizada). El análisis de estas asociaciones permitió definir ambientes depositacionales cuya sucesión e interrelación permite inferir un cambio progresivo en las condiciones de acomodación y paleoclimáticas imperantes durante su desarrollo. Resumen en inglés The continental deposits of the Punta San Andrés Alloformation (Plio-Pleistocene) crop out in the marine cliffs of south eastern Buenos Aires province (Fig. 1). The deposits of this unit have been assigned to different continental sub-environments such as floodplains, fluvial channels and temporary water bodies (Zárate, 1989). The aim of this work is to perform a high-resolution facies and facies association analysis as well as an architectural characterization of the s (mas) edimentary bodies cropping out in Baliza San Andrés and Complejo Turístico Chapadmalal (Fig. 1). This information will be used to elaborate a conceptual and dynamic model of the depositional environments present in the Punta San Andrés Alloformation and to define main controlling factors over its accumulation. The study area is located in a typical passive margin controlled by transversal extensional systems that originated rift basins (aulacogens) with very little basaltic magmatism (Ramos, 1996; Parker et al. 2008). This area experienced block rotation due to isostatic equilibrium and sedimentary overload. Accumulation of post-Miocene deposits was favored by a high sedimentary dynamic associated with the rise of the Andean Cordillera (Turic et al. 1996; Parker et al. 2008). More specifically, the study area is located in the southeastern part of the Tandilia Geological Province (Rolleri, 1975, Fig. 1). The pioneer papers on stratigraphy of the Mar del Plata and Miramar marine cliffs were carried out by Ameghino (1908), Frenguelli (1921) and Kraglievich (1952, 1953, 1959) (Figs.2, 3). The sedimentological characterization of these deposits was made by Teruggi et al. (1956) and Zárate (1989). This last author also proposed an allostratigraphic framework which intended to unify and simplify the late Cenozoic stratigraphy (Fig. 2). The rich vertebrate fauna present in the Punta San Andrés Alloformation and related units was studied since the first contributions by Ameghino (1908). These studies were the basis for the South American Late Cenozoic biostratigraphic scheme (Fig. 2; Marshall, 1985; Cione and Tonni, 1995, 1999). According to Zárate (1989), the Punta San Andrés Alloformation unconformably overlays the Pliocene continental deposits of the Punta Martínez de Hoz, Playa Los Lobos and Playa San Carlos alloformations. Its lateral extent is about 15 km from the Arroyo Lobería to Punta Vorohué (Fig. 1) and it also crops out between Arroyo Lobería and Playa San Carlos (Fig. 1). The mean thickness of the Punta San Andrés Alloformation is 15 meters. Internally, it is subdivided into three allomembers limited by discontinuity surfaces with a lateral extent that allows mapping them at 1:25.000 scale (Zárate, 1989; Fig. 2). Because of the high mineralogical and textural homogeneity (silty sand and reworked loessoid silt) of the cenozoic deposits in the study area (Teruggi et al., 1956) the discrimination amongst the different units is based on the abundance of the channelized facies, the carbonatic precipitation and the degree of development and maturity of paleosoils (Zárate, 1989). During field work 10 detailed, 1:50 scaled sedimentological profiles were made at Baliza San Andrés and Complejo Turístico Chapadmalal (Fig. 1) from which the 4 more complete are shown in figure 4. All stratigraphic logs were described in detail and focus was made in grain size, lithology, primary sedimentary structures, bioturbation, fossil content, scale and geometry of the lithosomes; all of these useful features to define sedimentary facies (Reading and Levell, 1996). The abundance of post depositional features (mainly pedogenetic) in the Punta San Andrés Alloformation made necessary the subdivision of sedimentary facies into two groups: depositional and post-depositional. The depositional facies were classified and described according to Miall´s scheme (1978, 2006), which was modified to facilitate their description and interpretation (Table 1). The post-depositional carbonatic facies were described according to Zárate´s scheme (1989) in which the studied features are the geometry of the calcareous accumulation (development in one, two or three dimensions), internal structure, hardness, abundance and lateral extent. The analysis of the seven depositional facies (Table 1) led to interpret a continental environment where unidirectional streams allowed the migration of 3D and 2D dunes as well as the lateral migration of side bars (facies Cig, ACt, ACs, Ah and Ae). It also made clear the presence of paleosoils developed on sheet-flood or splay deposits (facies Alp) and of suspension-related sediments (facies Pm). The analysis of the three post-depositional facies (Table 1) led to the identification of carbonatization processes related to pedogenesis (facies Bc and Tm) and to diagenesis in a vadose environment (facies Tp). The sedimentary facies were grouped into four facies associations (FA; Table 2). Facies associations I and II are related to fluvial channels and FA III and IV are related to floodplains. In the first group, fixed, lowsinuosity channels were differentiated from mobile, high-sinuosity channels based on their symmetric, simple ribbon geometry (Friend et al. 1979; Gibling, 2006) and their constituent facies (Cig, ACt and Ae) as opposed to multiepisodic, complex ribbons (Friend et al., 1979) with participation of Cig, ACt, ACs, Ah and Ae facies. The floodplain associations were divided in crevasse-splay and calcic paleosoils associations. The first one is composed of Alp facies, has a wedge-like geometry, a coarsening-upward arrangement, laterally related to fixed, low-sinuosity channels and a lateral extension over 3 kilometers (Figs. 10a, b). The former is mainly composed of Tp and Tm facies, has a tabular geometry, also laterally related to fixed, low-sinuosity channel and a lateral extension of hundreds of meters. Even though these four facies associations can be found all over the Punta San Andrés Alloformation, it is possible to identify three different sections (A, B and C, Figs. 9 and 11) in which their arrangement and proportions vary. Each of these sections is associated with specific depositional conditions that determined a characteristic fluvial system. Section A involves the lower and middle allomembers (Fig. 2). It is represented by floodplain deposits (both crevasse-splay and calcic paleosoils associations in a rhythmic arrangement) that are considered to represent a terminal fluvial system. The calcic paleosoils are interpreted to represent a dry and arid climate (Marriott and Wright, 1993; Balin, 2000) under a reduced rate of accommodation creation (Marriott and Wright, 1993; Cleveland et al., 2007). Because of the presence of iron nucleids and clay coatings, the crevasse-splay deposits are interpreted to represent wetter climatic conditions (Davies-Vollum and Kraus, 2001) and developed under a greater rate of accommodation creation (Cleveland et al., 2007). Section B (upper allomember) is also made up of floodplain deposits, although coarser and much more dissected by a larger proportion of complex channels. Hence, they are interpreted as a proximal floodplain. Finally, section C is represented by mobile, highsinuosity channel deposits in such a high proportion that in some parts of the outcrops there is no preservation of floodplain deposits. To summarize, for the Punta San Andrés Alloformation deposits cropping out in Baliza San Andrés and Complejo Turístico Chapadmalal, four facies associations can be proposed based on the facies analysis of the sediments and its geometry. The analysis of these associations allowed defining depositional environments whose interrelation reflected a progressive change in the paleoclimatic and accommodation conditions, probably related to base-level changes. For section A, climate is interpreted to have been dry and arid and related to low accommodation space. During deposition of section B, climate became wetter and accommodation space was increased. Section C is characterized by a lowering in the local base level that led to negative accommodation with fluvial incision. Deposition of channel fills and floodplain deposits indicate the return of positive accommodation space and wet climate.

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