Latin American journal of sedimentology and basin analysis

Suelos loéssicos influenciados por depósitos de conchilla pleistocenos de la formación pascua, Noreste de la provincia de Buenos Aires, Argentina

En el litoral de la provincia de Buenos Aires se encuentran varias unidades litoestratigráficas cuaternarias carbonáticas vinculadas a ingresiones marinas. En la Formación Las Escobas (Holoceno) se desarrollan suelos calcáreos litogénicos que han sido objeto de estudios taxonómicos, mineralógicos y micromorfológicos. La unidad marina más antigua es la Formación Pascua (Pleistoceno medio a superior) intercalada en la parte superior de sedimentos loéssicos de la Formación Pampiano y constituida por sedimentos arenosos, cuya parte superior posee alta concentración de valvas de moluscos fuertemente cementadas con carbonato de calcio, constituyendo una coquina. Los suelos actuales, de los cuales hay escasa información y son motivo de esta presentación, se desarrollan principalmente en los sedimentos suprayacentes de la Formación Pampiano y están afectados por procesos de carbonatación, vertisolización y argiluviación. Los objetivos de esta contribución son: a) analizar las propiedades de los suelos desarrollados en sedimentos loéssicos que cubren a la Formación Pascua, y b) estimar la influencia de los depósitos carbonáticos de esta formación en la génesis de los suelos. Se estudiaron cinco pedones (cuatro Molisoles y un Vertisol) en los partidos de La Plata, Magdalena y Punta Indio (provincia de Buenos Aires), en los cuales los depósitos carbonáticos tienen influencia en los horizontes inferiores de los suelos. Los pedones se desarrollan en sucesiones estratigráficas donde en la parte inferior se encuentra la Formación Pascua y donde los procesos de pedogénesis afectan a veces el techo de la misma. Es decir, que en algunos suelos hay un ciclo pedológico completo (horizontes A, B y C) desarrollado en materiales loéssicos y en otros casos, parte del ciclo pedológico (solum: horizontes A y B) se desarrolla en el loess y parte en la coquina (horizontes C). Todos los suelos poseen evolución calcárea, con horizontes enriquecidos con carbonato de calcio y/o horizontes cálcicos y/o horizontes petrocálcicos (calcreta). Los horizontes enriquecidos en carbonato de calcio y los horizontes cálcicos están desarrollados en los depósitos loéssicos, mientras que el horizonte petrocálcico se desarrolla en la parte superior de la coquina, por alteración epigénica de la misma, y cuyo contacto con el depósito loéssico suprayacente es neto (espesor <2 cm) y de forma plana u ondulada. Los horizontes petrocálcicos constituyen una calcreta pedogénica de clima húmedo, compacta, sin estructura (masiva) o laminar, de menos de 10 cm de espesor. El origen del carbonato pedogénico puede atribuirse a varios mecanismos: a) disolución de carbonatos en la parte superior del perfil, translocación descendente y precipitación; b) ascenso capilar y evaporación de agua carbonatada acumulada en la parte superior de capas compactas, casi impermeables; c) disolución y reprecipitación in situ de los carbonatos de la coquina y d) precipitación biogénica debida a la acción de bacterias, hongos y raíces, como proceso complementario. El proceso de carbonatación sería de origen mixto: a) por reorganización de la roca basal y b) por translocación en clima húmedo; en ambos casos el carbonato pedogenético tendría origen abiótico con participación biótica. Predomina la cristalización y recristalización, ya sea por disolución de depósitos preexistentes o por soluciones carbonatadas de desplazamiento vertical y lateral, interviniendo asimismo la biomineralización en estructuras orgánicas.

In the littoral area of Buenos Aires province two carbonate Quaternary lithostratigraphic units linked to marine transgressions are found. In the most recent unit (Las Escobas Formation, Holocene) lithogenic calcareous soils (Calciudolls and Rendolls) are developed, which have been studied in their taxonomic, mineralogical and micromorphologic aspects (Vargas Gil et al., 1972; Sánchez, 1976; Imbellone, 1996; Imbellone and Giménez, 1997; 1998). Conversely, little information on the soils associated to the oldest unit (Pascua Formation, middle to upper Pleistocene) is available. This unit is interbedded in the upper part of the loessial sediments of the Pampiano Formation, consisting of sandy sediments with high amounts of mollusk shells strongly cemented with calcium carbonate forming a coquina (Fidalgo et al., 1973). These deposits are overlain by sediments of the Pampiano Formation, over 2 m in thickness, where Mollisols and Vertisols have been affected by calcification, vertisolization and clay illuviation processes. The objectives of this article are: a) to analyze the properties of the soils developed in loessic sediments overlying the Pascua Formation, and b) to estimate the influence of the carbonate deposits of this formation in the soil genesis. Five pedons (four Mollisols and one Vertisol) located in the municipalities of La Plata, Magdalena and Punta Indio (Buenos Aires province, Argentina, Fig. 1) were studied. The soils have developed in an upland plain adjacent to coastal plains. The climate is temperate-humid, with a mean annual rainfall of 1040 mm, fairly well distributed; mean annual temperature is 16.2 oC (La Plata city, latitude 34o 55' S, longitude 57o 56' W, altitude 15 m a.s.l.). The monthly mean water balance shows a small deficit (7 mm) in summer and a substantial surplus (240 mm) between autumn and spring. The soil moisture regime is udic and the soil temperature regime is thermic. According to the classification of Thornthwaite (1948) the climate of the area is B1 B´2 r a´. Native vegetation is dominated by grasslands, largely modified by agriculture and grazing. The studied soils are polygenetic due to the geomorphologic evolution of the area. The sedimentary deposits include: a) transgressive deposits of the Querandinense ingression (humid climate) at the base; b) Postquerandinense eolian deposits (dry climate) and c) Postplatense deposits (dry climate) in the upper part. Pedons 2, 3, 4 and 5 have developed in loessial sediments. Unlike other carbonatic soils of the coastal plains, they are practically devoid of primary carbonate. On the other hand, Pedon 1 would have developed in a mixed deposit: an upper soil including shell fragments (Holocene), overlying a calcrete formed in a Pleistocene loessial deposit (Fig. 2, Table 1). The presence of pedogenic carbonate is revealed by macromorphological features such as pseudomycelia, soft powdery masses, some concretions and diffuse accumulations in root pores in the upper part of the solum, whereas the horizons overlying the rock contain lithogenic calcareous pebbles and shell fragments, as well as some pedogenic carbonates (soft powdery masses). Pedons 2, 3, 4 and 5 have a similar distribution of CaCO3 equivalent in depth. The carbonate is absent or has <1% in the upper horizons, it increases in the middle horizons and may reach 70% at the soil base or 80% in the coquina. The morphological and chemical characteristics of the calcareous accumulations allow the following horizons to be differentiated: a) calcium carbonateenriched horizons (all pedons); b) calcic horizons (pedons 4 and 5) and c) petrocalcic horizons (pedons 1, 3 and 4). All of them exhibit calcareous reorganization. The former two are developed in the loessial deposits, whereas the latter are developed in the upper part of the coquina by epigenic alteration. Its contact with the overlying loessial sediments is sharp (<2 cm in thickness), with level or undulating shape, forming a compact, structureless (massive) or platy calcrete, less than 10 cm thick. The carbonate-enriched and calcic horizons have randomly distributed pedogenic acicular calcite (Fig. 3); the former with <15% and the latter with ≥15% of CaCO3 equivalent. The petrocalcic horizons are massive or platy with ordered tabular calcite and rhombohedra and scalenohedra in rock holes, where CaCO3 equivalent can be as high as 60-70%. Grain-size distribution in pedons 2, 3 and 4 is similar and typical of the soils developed across the Undulating Pampa. The histograms show a welldefined mode in the coarse silt fraction (62-32 µm, 4-5 Ø) ranging from 24.01 to 25.34% in the eluvial horizons and from 14.17 to 22.42% in the illuvial horizons (Fig. 4). There is also a high amount of <1 µm clay (below 10 Ø), with 19.32-20.18% in the eluvial horizons and 22.90-52.17% in the illuvial horizons. The sand fraction is much lower, with fine and very fine sand as the dominant subfractions. The grain-size distribution of pedon 1 is almost uniform up to the loess deposit because illuvial horizons are absent. The mineralogy of the clay fraction in the loessial portion of Vertic Argiudolls (pedons 2 and 3) is mainly illitic, with well-defined reflections of this mineral and subordinate amounts of expandable minerals and kaolinite (Fig. 5). In addition to this general trend, a marked increase of expandable clays in B horizons was observed with respect to the A horizons. This would suggest, together with grainsize variations in the coarse fraction, a sedimentary discontinuity; thus, the soils of the area would have formed in two loess mantles with different grain-size composition and clay mineralogy. Micromorphological features revealed through optical microscopy include micritic and acicular calcite coatings and micritic nodular concentrations. SEM reveals recrystallization and dissolution morphologies in calcite crystals, which sometimes are found in the same horizon (Figs. 5, 6, 7). The pedons exhibit different crystallization morphologies; in the calcrete the cement has more or less equidimensional "rice grain"-shaped crystals and sparry and microsparitic calcite, whereas in the loessic horizons acicular calcite with variable morphology is mainly found. In the studied soils, acicular calcite presents different morphologies which may coexist in the same horizon: a) masses of disarranged crystals, 10- 50 µm long and 1 µm wide; b) needles with serrated irregular borders, c) complex morphologies of needles with serrated borders and irregular growths on the needles and, d) rhombohedric calcite en échelon. The calcrete in the petrocalcic horizons may present a laminar zone with tabular and rhombohedric calcite. The origin of pedogenic carbonate can be ascribed to various mechanisms, with different degree of influence: a) dissolution of carbonates in the upper part of the profile, downward translocation and precipitation; b) capillary rise and evaporation of carbonate water accumulated on the top of compact, almost impervious layers, c) in situ dissolution and reprecipitation of the coquina carbonates (petrocalcic horizons) and, d) biogenic precipitation due to the action of bacteria, fungi and plant roots, as a complementary process. The origin of the calcification process in the studied soils would be a mixed reorganization of the base rock and translocation under humid climate. In both cases the pedogenic carbonate would have had an abiotic origin with some biotic participation. Crystallization and recrystallization are dominant, either through dissolution of pre-existing deposits or through carbonate solutions moving vertically and laterally; biomineralization in organic structures has also an influence. In this way, the petrocalcic horizon is a pedogenic calcrete of humid climate.

Hydrosedimentological studies in the paciencia dam, southeastern Brazil

The utilization of dams for many purposes, such as water supply, navigation, flood control, irrigation, storage of water and generation of electricity, among others, solves several problems but unfortunately creates many others. Water management is not a simple task, and the presence of dams generates social, physical, chemical, biological and environmental consequences. Among these severe impacts associated to the implantation and operation of dams, the issues related to the transported and deposited sediments are very significant, although many times hard to evaluate. The objective of this paper is to present the results of studies performed close to the dam of a small hydro power plant located in the Paraibuna River, in the southeast part of Brazil. A grain size analysis of the sediments and its chemical characterization were evaluated before and after a bottom discharge. Two bathymetric surveys in a section of 1.6 km upstream the dam were also performed, one just before and other just after the bottom discharge, in order to evaluate its influence in the bathymetry of the river. The results showed that after the bottom discharge, the amount of muddy sediments upstream the dam became smaller. Regarding to the heavy metal content, the bottom sediments collected upstream and downstream the dam presented, in several samples, values higher than the threshold above which a probable adverse effect on the biota is expected, in particular for cadmium, chromium and zinc, in order of importance.

A utilização de barragens para diversos fins, tais como abastecimento e armazenamento de água, navegação, controle de cheias, irrigação e geração de energia elétrica, entre outros, resolve vários problemas, mas, infelizmente, cria muitos outros. A gestão da água não é uma tarefa simples, e a presença de barragens gera consequências sociais, físico-químicas, biológicas e ambientais. Entre os vários impactos associadosà implantação e operação de barragens, as questões relacionadas ao transporte e deposição de sedimentos são muito significativas, embora muitas vezes difíceis de avaliar. O objetivo deste trabalho é apresentar os resultados de estudos realizados próximo à barragem de uma pequena usina hidrelétrica localizada no Rio Paraibuna, na parte sudeste do Brasil. A granulometria dos sedimentos e sua caracterização química foram avaliadas antes e após uma descarga de fundo. Dois levantamentos batimétricos em uma seção de 1.6 km a montante da barragem foram também realizados, um logo antes e outro logo após a descarga de fundo, para avaliar a sua influência na batimetria do rio. Os resultados mostraram que, após a descarga pelo fundo, a quantidade de sedimentos finos a montante da barragem tornou-se menor. No que se refere à presença de metais pesados, os sedimentos de fundo coletados a montante e a jusante da barragem apresentaram, em diversas amostras, valores superiores ao limiar acima do qual efeitos adversos na biota são esperados, em particular para o cádmio, chumbo e zinco, em ordem de importância.

Evolución paleoambiental de cordones litorales holocenos durante una caída del nivel del mar en la Bahía Samborombón, Buenos Aires, Argentina

Las grandes fluctuaciones climáticas cíclicas ocurridas durante el Cuaternario tuvieron un efecto muy marcado en las regiones costeras, afectando principalmente el ambiente geomorfológico y la sedimentación de los depósitos allí acumulados. El mejor entendimiento de los sistemas depositacionales litorales generados en el pasado más reciente representa una herramienta fundamental para reconstruir y comprender otros depósitos similares del registro geológico, así como para desarrollar predicciones sobre posibles escenarios de cambios climáticos futuros. En el área costera de la Bahía Samborombón, noreste de la provincia de Buenos Aires, se encuentran preservados excelentes depósitos litorales acumulados con posterioridad al Último Máximo Glacial, durante la transgresión holocena. Sobre la base del análisis de facies se reconocieron cinco unidades de acumulación: cordón arenoso, cordón bioclástico, depósitos de sobrelavado, planicie de mareas y albufera. El análisis arquitectural y la jerarquización de superficies de discontinuidad permitieron elaborar un modelo de acumulación dinámico de los cordones de playa y ambientes asociados en diferentes etapas durante el Holoceno (ca. 8000 a A.P.- presente). El análisis estratigráfico secuencial permitió establecer la interacción de dos controles alocíclicos predominantes: eustático (regresión forzada) y climático (Óptimo Climático del Holoceno medio). La interacción de ambos factores extrínsecos no sólo condicionó la acumulación y amalgamación de geoformas cordoniformes, sino también, la relación entre aporte terrígeno y productividad carbonática de los depósitos.

In the Argentinean littoral zone several transgressive - regressive events took place during the Quaternary. Excellent examples of these events are recorded in the Bahía Samborombón coastal area in northeastern Buenos Aires province (Fig. 1). Following the stratigraphic scheme of Fidalgo et al. (1973) modified by Fucks et al. (2010), deposits of the Canal de las Escobas Formation (Holocene) were analyzed (Figs. 2, 3). The Canal de las Escobas Formation was discriminated into 4 members (Fucks et al., 2010), the Destacamento Río Salado Member (tidal flat, 5.8-7 ka B.P.), the Cerro de la Gloria Member (beach ridge, ca. 3-8 ka B.P.), the Canal 18 Member (coastal plain, 6-7 ka B.P.) and the Canal 15 Member (coastal plain, 3 ka B.P. to present). Fourteen detailed sedimentary sections were logged in the study area (Fig. 4), taking into account lithology, primary sedimentary structures, geometries of the sedimentary bodies with their orientation and the hierarchy of the bounding surfaces, as well as the palaeontological (predominantly molluscan) content. Thirteen sedimentary facies were defined (Figs. 5, 6) which were grouped into three major divisions: siliciclastic sandstones, fine-grained siliciclastic and carbonatic bioclastic rocks. The excellent preservation of the Canal de las Escobas Formation in the study area allowed the recognition of five sedimentary units based on the spatial facies arrangement and the hierarchy of internal surfaces. These units are: sand and bioclastic ridge, washover, lagoon and coastal plain units (Figs. 7, 8, 9, 10, 11). In order to determine the palaeoenvironmental evolution, based on the stacking patterns of the sedimentary units together with the hierarchy of the sedimentary discontinuity surfaces, six stages were devised after the maximum Holocene transgression (Fig. 12). Stages 1 and 2 correspond to the development of a sand ridge as a response to the erosion and reworking of a previous spit system (Fig. 3; Facies Pinamar). Subsequently, during stages 3 to 5, a bioclastic ridge was amalgamated with the sand ridge (Fig. 12). Simultaneously to these five stages, in a landward position, lagoon and washover deposits were developed (Fig. 12). Finally, during stage 6 the continuous sea level fall generated the actual coastal plain deposits (Fig. 12). The sudden change in the ridge deposits composition, from sandy sediments to almost exclusively bioclastic (carbonate) between stages 2 and 3 can be interpreted as the combined result of an increase in carbonate productivity along with a decrease of siliciclastic supply together the coast. This stage would have been developed approximately 5-6 ka B.P. (Marine Isotope Stage, MIS, 1), in coincidence with the Holocene Climatic Optimum or Mid-Holocene Thermal Maximum (sensu Briner et al., 2006) during which the very special climatic conditions may have led to the proliferation of large communities of benthic organisms, particularly bivalve mollusks, which are not so abundant today. The high occurrence of Mactra isabelleana in the beach ridges of Bahía Samborombón, including all dimensions and ontogenetic stages and excellent preservation of the shells, indicates that this bivalve represents an autochthonous element of the coastal native communities, suggesting high adaptive radiation of this species in the area and time span considered, when atmospheric-oceanic circulation patterns / changes in the prevalent shallow oceanic currents are documented by different sources of evidence (palaeobiodiversity, palaeobiogeography, stable isotopes) and the warm Brazilian current (moving southwards) prevailed over the cold Malvinas current (flowing northwards) (Fig. 3). This change in the intensities of the ocean currents would have originated a dual effect: firstly, a greater contribution of warm-temperature waters to the bay and secondly, the change in the terrigenous input. The contribution of sand, due to erosion and reworking of the previous spit facies, would have drastically decreased, whereas the contribution of terrigenous may have been restricted from the north coming from the Río de la Plata (Fig. 3). Nevertheless, no changes in composition and/or texture were observed in siliciclastic material between the sand and bioclastic ridges units. This would be related to the sort of material transported by the Río de la Plata, which is mostly pelitic and potentially washed by tidal activity as it is well documented for similar Quaternary deposits from other areas (Meldahl, 1995). Considering the whole dataset gathered, we propose an alternative sequence stratigraphic scheme for the Holocene ridge development from Bahía Samborombón (Fig. 13). This ridge is now considered originated as a consequence of a forced regression (ie. part of a falling stage system tract) instead of the previously interpretation during the late stage of sea level rise (ie. part of a highstand system tract). Both eustatic and climatic controls can be recognized, although with different orders of magnitude, which controlled the genesis of the studied deposits. In turn, this study provides an example of the strong changes occurred in coastal environments as a result of climate change, particularly in the context of global warming episodes which characterized interglacial periods of the Quaternary in South America (e.g. MIS 1, 5, 11).

Aspectos tefrológicos de la erupción del volcán Quizapú de 1932 en la región de la Laguna Llancanelo, Payenia (Mendoza, Argentina)

El Volcán Quizapú es parte del Complejo Volcánico Cerro Azul-Descabezado Grande, ubicado en la Provincia de Talca, Chile (36,67°S - 70,77°O, altura máxima: 3788 m s.n.m.). La erupción del 10 de abril de 1932 fue uno de los mayores eventos volcánicos del siglo XX. Tuvo un carácter pliniano y arrojó un volumen de tefras entre 5 y 30 km³ (según diferentes autores), que por efecto de los vientos dominantes del oeste cubrieron gran parte de la región central de Argentina, llegando a la costa atlántica y afectando a otros países del este de Sudamérica. Los efectos climáticos y el impacto en las regiones más proximales del sur de Mendoza, particularmente en el Departamento de Malargüe, fueron muy significativos. El estudio de los eyectos constituye un campo de exploración de gran valor no solamente para conocer las características, alcances y efectos de esa erupción sino también para evaluar aspectos relacionados con la tefrología. En esta contribución se analiza un depósito de tefras en los alrededores de la Laguna Llancanelo, en las cercanías de Malargüe, una de las áreas más afectadas por la erupción. Las determinaciones sedimentológicas, mineralógicas y texturales (petrografía, microscopía electrónica y determinaciones químicas semicuantitativas con EDS) permitieron caracterizar la composición granulométrica, petrográfica y química semicuantitativa de las tefras. Estas características son afines a las de los materiales piroclásticos eyectados por la erupción del volcán Quizapú de 1932 estudiados por otros autores, por lo que se asignan a dicho evento volcánico. Las tefras depositadas en la zona de estudio son de tamaño arena muy fina a mediana con significativa cantidad de fracciones menores a 10 µm. Las trizas son pumíceas, fibrosas, con diferentes conformaciones morfológicas y abundante vesicularidad que favorece el entrampamiento de partículas menores en las vesículas de las mayores. La composición química revela un alto contenido de sílice que alcanza hasta cerca del 70% de los componentes, con alrededor de un 15% de Al y cantidades subordinadas de K, Na, Ca, Zn, Mg, Cu, Fe y Ti. Es notorio el alto contenido de K, asociado a un aumento relativo por desilicación de la tefra con el transcurso del tiempo. También son importantes los contenidos de Fe y Cu, en este último caso posiblemente asociado a transformaciones post-depositacionales por meteorización. La composición de las trizas señala una erupción de tipo pliniano magmática andesítico dacítica con contenido hidromagmático, intensa fragmentación y aglutinación durante el enfriamiento. Las evidencias de campo demostraron que la erupción afectó severamente la región sepultando y quemando suelos, carbonizando vegetación y cegando sectores marginales de la laguna. Estudios de esta naturaleza aportan a una clasificación sistemática y comparativa de la peligrosidad volcánica, considerando que las tefras resultantes de la erupción del Quizapú contienen hasta un 10% de partículas PM 10 (tamaño <10 µm) que corresponden a los tamaños "respirables" y en consecuencia altamente nocivos para la salud. Desde el enfoque socioeconómico, estos estudios sirven de base para su aplicación a otros eventos eruptivos muy recientes ocurridos en Argentina (volcanes Hudson, Copahue, Chaitén, Llaima, Peteroa y Puyehue-Cordón Caulle), y permiten elaborar metodologías específicas para analizar la peligrosidad de futuros eventos volcánicos.

Tephrology is a broad term that comprises all the aspects related to "tephra" studies (stratigraphy, chronology, petrology, sedimentology, chemistry, Froggat and Lowe, 1990; Lowe and Hunt, 2001) (Fig. 1). In Argentina, tephrological studies have significantly increased recently as a result of the increment in the Southern Andes volcanic activity affecting the country in the last two decades (E.g.: Corbella et al., 1991a,b; Stern, 1991; Mazzoni and Destéfano, 1992; Nillni et al., 1992; Gonzalez Ferrán, 1993; Naranjo et al., 1993; Scasso et al., 1994; Nillni and Bischene, 1995; Haberle and Lumley, 1998; Villarosa et al., 2002; Kilian et al., 2003; Naranjo and Stern, 2004; Orihashi et al., 2004; Stern, 2004; Scasso and Carey, 2005; Daga et al., 2008; Watt et al., 2009; Martin et al., 2009; Leonard et al., 2009; Rovere et al., 2009, 2011; Wilson et al., 2009, 2012). The eruption of Quizapú volcano (Volcanic Complex Azul-Descabezado Grande, Province of Talca, Chile, 36,67°S-70,77°W, maximum height of 3788 m a.s.l.), that occurred on April 10, 1932, represented one of the largest eruptions worldwide in the 20th Century. It affected extensive regions of Argentina as well as many coastal areas of the Southwestern Atlantic Ocean as a result of the prevailing westerly winds, and specifically impacted dramatically in regions located nearby the source volcano (Department of Malargüe, Province of Mendoza, west-central Argentina, Fig. 2). The wide spreading of the resulting tephras and its easy reconnaissance in the field provides a great opportunity for detailed studies about the eruption and its products. Results on the eruptive aspects and tephras dispersion and deposition from this eruption were published by some authors (Lunkenheimer, 1932; Kittl, 1933; Walker, 1981, Hildreth and Drake, 1992, González Ferrán, 1993; Ruprecht and Bachmann, 2010; Ruprecht et al., 2012). In this contribution the sedimentological, mineralogical and chemical characteristics of the tephra deposits occurring at the Llancanelo Lake and surroundings, located 140 km east (downwind) of the Quizapú volcano, are studied based on grain-size, petrographic and electron microscope analysis (SEM) as well as semiquantitative chemical determinations by Energy Dispersive Spectrometer (EDS). The obtained results, when compared with the results of analyses performed by other authors in tephras from the 1932 eruption of the Quizapú volcano, allow attributing the studied tephra layer to this eruption. On these bases, diverse aspects related to the depositional and post-depositional aspects of the tephras are herein discussed, as well as some environmental changes produced by the eruption. On the other hand, this paper contributes to a systematic and comparative classification of volcanic hazard in health and society that serves as base-studies for better understanding other more recent Southern Andes eruptive events that affected Argentina (Hudson, Copahue, Chaitén, Llaima, Peteroa and Puyehue-Cordón Caulle volcanoes). The eruption of Quizapú volcano in 1932 was one of the most important events among a long history of activity of this volcanic complex (Smithsonian Institution, 2012). It had a plinian character and threw into the atmosphere enormous amounts of tephras varying between 5 and 30 km³ according to different authors (Kittl, 1933; González Ferrán, 1993; Hildreth and Drake, 1992; Ruprecht and Bachmann, 2010), producing a dramatic impact in society, agriculture and local economies in the downwind neighboring affected regions (Abraham and Prieto, 1993; González Ferrán, 1993). The tephra deposits were very uniform in thickness with a notable decreasing grain-size tendency with distance from the source volcano, ranging from 6 cm in neighboring areas and reaching silt and clay sizes around 100 km east (Kittl, 1933; Hildreth and Drake, 1992). The horizon of tephras was recognized as a regional level in a number of natural outcrops pits and excavations, as well as in sediment cores recovered from short drillings (Fig. 3). The tephra level was affected by compaction and post-depositional transformations after 80 years of burying and exposure to weathering and pedogenetic processes, although most of the original characteristics are very well preserved. The sedimentary sequence in which the tephra level is included was recognized regionally by surface and subsurface surveys based on geoelectrical methods and short drillings (Violante et al., 2010; Osella et al., 2010, 2011; de la Vega etal., 2012). The sequence is composed of light brown sandy-silty sediments of lacustrine and eolian origin with high volcaniclastic content and interbedding of buried soils and evaporites (Rovere et al., 2010a,b; D´Ambrosio et al., 2011). In some profiles (P19 and P42, Fig. 3) located in marginal areas east of the lake, the tephra layer overlies lacustrine deposits and is in turn covered by eolian deposits; this indicates that the lake borders were filled with tephra during the eruption and definitively desiccated, and were later covered by eolian deposits probably as a result of the aridity of the climate that followed the eruption. On the other hand, in the lacustrine plain west of the lake the tephra layer was not found; a possible explanation for this is either post-depositional erosive processes or not deposition, as some places could have been, at the moment of the eruption, part of the lacustrine body with higher water energy, and therefore the ash was dispersed without leaving any recognizable deposit. Northwest of the lake, the tephra deposit was found overlying a buried soil containing burned vegetation remains (profile P45, Fig. 3), suggesting high temperatures of the ash fall with consequent burning of vegetation, as it was also documented in other regions of the world (Carson et al., 1990; Seymour et al., 1993). In the lacustrine coastal plain of the lake, tephra layers were found overlying eolian deposits (profiles P5, P21 and P26, Fig. 3). Tephra´s grain-size indicates varied sizes between very fine and medium sand. Sediments are poorly sorted and statistical grain-size distributions (Table 1, Fig. 4) are bimodal with two well-marked populations separated at the size-range of 3-3,5 (88- 125 µm). Population 1 is coarser with mode between 1 and 2 (250 to 500 µm), whereas Population 2 is finer with mode between 4 and 7 (63 to 8 µm). This bimodal distribution is typical for distal tephras (Bonadonna and Houghton, 2005; Rose and Durant, 2009). The lower-sized population contains the "respirable particles" (PM10 <10 µm, Horwell et al., 2003, Horwell and Baxter, 2006). Optical microscopy allowed obtaining the bulk mineralogical composition and details of the ash shards. Bulk composition is: 59% volcanic glass, 40% crystals (in decreasing order: plagioclases, magnetite, hornblende, pyroxenes, quartz, olivine and ilmenite) and 1% lithoclasts (possibly andesitic volcanic pastes). Glass is mainly composed of fibrous, pumiceous shards with vesicular microcavities, most of them tubular and elongated with minor amount of cuspate, blocky and platy individuals (Figs. 5, 6 and 7). Besides, the minerals contain vesiculated glass adhered to the crystals. SEM analyzes were aimed at observing details of the particle´s shapes and surface characteristics. They are all of varied shapes ranging from equidimensional, elongated (prismatic) and irregular, from rounded to angular with sharp edges, with striations and different degrees of vesicularity (Figs. 6 and 7). Glass shards show a major composition of light brown glass (possibly sideromelano) although dark glass is also present, and they show some coating. Its vitreous textures were defined following the clasification by Miwa et al. (2009), as massive with two types of surfaces, smooth-uniform (S-type) and not-smooth-irregular (NS-type) with alveoli and hollows (Fig. 7). The coating consists of highly cohesive small particles (<25 µm, and hence they correspond to the "respirable" sizes) which can be partially adhered by some melting process to the larger particles. EDS revealed predominance (in decreasingorder) of SiO2 (up to ~70%), Al2O3 (up to ~15%), with lesser amounts of K, Na, Ca, Zn, Mg, Cu, Fe y Ti (Fig. 7, Table 2). The three last mentioned components are abundant as oxides included in the ash. K is an important component in accordance to the high K content of the Volcanic Complex Cerro Azul - Descabezado Grande - Quizapú (Backlund, 1923), which seems to have been proportionally increased in percentage by desilication of the tephra during transport (Aomine and Wada, 1962). On the other hand, high concentrations of Cu were found in some samples (Fig. 8, samples P5 III and P20 I in Table 2), what is preliminary associated to postdepositional alteration of tephras by weathering and transformation in alofana and halloysite with incorporation of high Cu content. The sedimentological and semi quantitative chemical characteristics of the studied tephras from Quizapú eruption, together with the erupted volume of tephras and the volcanic column height mentioned in the available bibliography, are compatible with an explosive plinian eruption (Walker, 1981; Newhall and Self, 1982; Simkin and Siebert, 1994; Bonadonna and Houghton, 2005; Rose and Durant, 2009; Carey et al., 2009; Gislason et al., 2011; Smithsonian Institution, 2012). This eruption seriously affected the southern Mendoza province where Llancanelo lake is settled, producing a reduction of the lake size, the burying and burning of soils and the increasing in aridity of the region. These effects can be easily observed in the field according to the stratigraphic relations of the Quizapú Volcano tephras level with the under- and overlying lacustrine, eolian and buried soils levels. The eruption caused the collapse of the local farming, agriculture and livestock economies as well as heavily impacted in society. The obtained sedimentological, mineralogical, petrographical and chemical characteristics of the tephras reveal fractioning processes during the eruptive and post-eruptive phases with deep posteruptive changes in the particles concentrates, following the concepts by Rose and Durant (2009). Additional complications to the resulting tephras deposits arise from aggregation processes, as it was documented in Chaitén Volcano tephras erupted in 2008 (Watt et al., 2009). Agglutination of particles also occurred, possibly as a result of primary salts formed by exsolution during the aerial transport and deposited as coatings on the particles surfaces, that later reacted in contact to atmospheric fluids (Delmelle et al., 1980, 2007, Gislason et al., 2011); however, preservation of such coatings is unlikely due to post-depositional processes such as dissolution, weathering, alteration by phreatic activity, secondary recrystallization, etc. Particles surface features reveal two types of textures following the concepts by Miwa et al. (2009), which reveal eruptive characteristics, energy of the transport process, gases content and post-depositional processes. SEM analysis show typical characteristics of an andesitic magmatic eruption or partially fluid with hydromagmatic components. Particles morphology and the thickness of vesicle´s walls would preliminary indicate relative fluidity and medium viscosity, as well as the cooling velocity in the volcanic conduit. Smaller particles (finest fractions of Population 2) adhered and partially cemented to the larger particles would indicate pulverization during fragmentation (Wohletz and Krinsley, 1982). Particles containing high Cu proportions are thought to have been produced by transformation of volcanic glass in alofana and hydrated halloysite during weathering, what results in desilication and increasing Cu content (Fig. 8), although further studies are needed on this matter. On the other hand, the SiO2-K 2 O relation (Fig. 9) arranges the samples in a graphic field close to that reported by other authors (Fierstein et al., 1989; Hildreth and Drake, 1992; Ruprecht et al., 2012), although with a slight decreasing in Si content that is associated to differential particles deposition according to the distance from the source volcano and desilication. The reduction in the lake size evidenced by the regional geology and stratigraphic sequences (Violante et al., 2010; de la Vega et al., 2012) as well as the burning of vegetation underlying the Quizapú tephras layer, agree with oral versions from aged inhabitants of the region, who mentioned the fallout of "hot" ash and the drying of large lacustrine areas during the months that followed the eruption (Ovando and Ramires, 2009). Both the reduction in the lake size as well as the capping of the tephra layer by eolian deposits also demonstrate the aridity of the region that followed the eruption, as stated by Abraham and Prieto (1993). The volumes of ejected fine tephras of sizes smaller than 10 µm, and particularly those smaller than 4 µm, which are considered to affect human health as they can produce respiratory diseases -particularly if they have high silica content and sharp-shaped (Horwell et al., 2003)-, reveal the potential harmful of these materials. Grain-size distributions of Quizapú tephras show around 6% of particles <10 µm. If it is considered the 150 ton km-2 of tephra released by the eruption (according to the estimations by González Ferrán, 1993), hence about 9 ton km-2 of respirable particles could have been incorporated into the atmosphere, from which 35% (3.15 ton km-2) is even lesser than 4 µm in size. These numbers must be taken into account in order to evaluate health impact. Studies of this kind contribute to develop methodologies in tephrological analysis to be applied to other recent eruptive events and for evaluating in a multidisciplinary way the volcanic hazard on environment and society.