Scielo RSS <![CDATA[Latin American journal of sedimentology and basin analysis]]> http://www.scielo.org.ar/rss.php?pid=1851-497920130002&lang=en vol. 20 num. 2 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <![CDATA[Case studies on the quaternary and geomorphology of Argentina: preface]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792013000200001&lng=en&nrm=iso&tlng=en <![CDATA[Sedimentology and stratigraphy of upper Miocene deposits from the transverse valleys of La Pampa province, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792013000200002&lng=en&nrm=iso&tlng=en En la región centro-oriental de la provincia de La Pampa, comprendida en el ámbito del Bloque de La Pampa Central, los depósitos de la Formación (Fm.) Cerro Azul constituyen el registro de la sedimentación continental neógena de antepaís (Mio-Plioceno). La unidad aflora en los márgenes y pisos del sistema de valles transversales que domina el paisaje del área analizada. En el interior de los valles aparecen comúnmente depósitos dominantemente limosos, no consolidados, descriptos como facies lacustres basales de la Fm. Cerro Azul. Conforman, en general, lomadas de hasta 5 m de altura relativa que en varias localidades marginan los sectores más deprimidos de los valles. El análisis de estos depósitos ha permitido plantear interrogantes respecto a su posición estratigráfica y ambiente de sedimentación. Entre ellos, ¿los depósitos corresponden a las secciones estratigráficas más antiguas de la Fm. Cerro Azul? ¿o representan unidades de agradación desarrolladas en el interior de los valles transversales, en forma posterior a su incisión, en algún momento del Mioceno tardío? ¿Corresponderían a ambientes de sedimentación que se han repetido en el Mioceno tardío, dando lugar a depósitos de características litológicas similares en distintas posiciones estratigráficas de la sucesión de la Fm. Cerro Azul?. Para dilucidarlo se llevó a cabo un estudio sedimentológico-pedológico, estratigráfico y geomorfológico sobre dichas facies limosas. Éstas registran ambientes sedimentarios de tipo fluvial efímero, de baja energía, y de tipo palustre, donde habrían ocurrido procesos de sedimentación y pedogénesis. Estos últimos habrían sido particularmente dominantes durante algunos lapsos de la evolución de estos sistemas. La posición y expresión geomorfológica que exhiben permite plantear que son parte de un ciclo de agradación acontecido en el interior de los valles transversales en algún momento de su evolución durante el Mioceno tardío.<hr/>In the distal foreland region of central Argentina the Mio-Pliocene continental record is widely distributed (Fig. 1a, Folguera and Zárate, 2009). In La Pampa province it is represented by the Cerro Azul Formation (Fm) (Linares et al., 1980; Visconti, 2007) that involves the lapse between ca. 12-11? Ma and 5.8 Ma (Verzi and Montalvo, 2008; Folguera and Zárate, 2009). This lithostratigraphical unit crops out along the walls and floors of the transversal valleys (Calmels, 1996) situated in the central-eastern sector of La Pampa province. The deposits are blanketed by aeolian silty sands and sandy silts mantles which together with the sand dunes systems usually located along the transversal valleys are considered of Late Pleistocene-Holocene age (Zárate y Tripaldi, 2012). According to Visconti (2007) the outcrops of the Cerro Azul Fm consist of lacustrine, aeolian and fluvial facies. The lacustrine facies, made up of laminated claystones and sandy siltstones to very fine sandstones, have been stratigraphically placed at the base of the Formation. However, field work allowed recognizing the existence of similar deposits making up comparable geoforms at different absolute altitudes in several transversal valleys. This paper is focused on the sedimentological characteristics of these deposits and questions their stratigraphic position. Based on the observations and previous work, three working hypothesis are considered: the deposits are the oldest exposed stratigraphic sections of the Cerro Azul Fm as already stated; the deposits post date the valleys formation, hence they were accumulated within the valleys; the deposits represent similar and diachronic sedimentary settings during the accumulation of the Cerro Azul Formation. The study consisted of a sedimentological-pedological description and sampling coupled with the geomorphological analysis of the deposits at several localities in different transversal valleys of La Pampa (Fig. 1b). Textural, mineralogical (X-Ray difractometry) and micromorphological analyses of thin sections were carried at selected samples. A lithofacies approach was used to describe the deposits resulting in the identification lithofacies (Table 1) and facies associations following Miall's (2006) proposal, with a partially modified lithofacies code. Interpretations were made following Miall (2006), Reading (1996) and Nichols (2009). Two main geomorphic systems (SE1 and SE2) of different altitude are identified in the Nerecó, Hucal, Quehué and Chapalcó valleys (Figs. 1b, 2a) resulting in a characteristic stepped topography. The deposits under study compose the SE2 with a relative height between 2 and 5 m above the valley floor, surrounding the deepest part of them; the lower stratigraphic contact remains unexposed. They are made up of unconsolidated sandy silt, silt and, secondarily, silty fine sand, either massive or with rare planar laminar bedding at some localities, while a dominant angular blocky structure is present at exposed sections. In some exposures, the deposits under study are featured by the occurrence of cutans and slickensides. Calcium carbonate is commonly present as powdery accumulations, nodules and concretions. The organic matter content varies from 0.03 to 1.17 %. At some levels the deposits exhibit layers of darker colors and higher concentration of organic matter with subprismatic secondary structure when dried. According to the grain-size and primary and secondary structures several lithofacies have been identified: Afm(f), Afb, Lab(c), Lab(mf), Lam(i), Lb(fc), Lal, Lab(cP), Afb(mfP) (Table 1), which were grouped into two main facies, A and B. Facies A represents abandoned channels of low relief and nonchannelized areas linked to low energy ephemeral streams/flows transporting sediments to the deepest part of the landscape. Facies B is interpreted as a paludal-like setting corresponding to poorly drained low energy aggradational environments. Both facies are gathered into facies association 1 (FA1), recognized in all the studied localities. It is interpreted as the record of low energy ephemeral fluvial and paludal-like environments, including episodes of higher water inputs linked to heavy rain events. This together with the geomorphological setting of the deposits suggests the occurrence of an aggradational process of low energy during the late Miocene in the interior of the valleys once they were already formed. <![CDATA[Paleoenvironmental analysis and interpretation of upper Quaternary sequences in summit planation surfaces of center-south of Sierra de Comechingones, Córdoba, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792013000200003&lng=en&nrm=iso&tlng=en Las pampas de altura presentes en Sierras Pampeanas corresponden a paleosuperficies erosivas labradas sobre rocas de basamento precámbrico-paleozoico inferior, fracturadas, elevadas y basculadas durante la Orogenia Ándica, y posteriormente sometidas a nuevos ciclos denudativos. Estas paleosuperficies están parcialmente cubiertas por sedimentos cenozoicos, en general secuencias loéssicas/loessoides cuaternarias, con espesores variables entre 0,5 y 5 m, las cuales hasta el presente han sido poco estudiadas. El objetivo de este trabajo es analizar los depósitos de la pampa de altura de la Sierra de Comechingones, entre los 32º42´ y 32º50´S y los 64º52´ y 64º59´O, con el propósito de establecer condiciones paleoambientales-paleoclimáticas, y proponer una secuencia estratigráfica preliminar para el área. Se describieron y muestrearon tres perfiles representativos y se efectuaron determinaciones cronológicas, mineralógicas, granulométricas y de materia orgánica. Se diferenciaron tres unidades estratigráficas. La unidad I resulta franco limosa, moderadamente seleccionada, con una edad OSL de 23,0± 2,25 ka. En contacto neto sobreyace la unidad II, formada por materiales dominantemente franco arcillosos, con estructuras de corte y relleno, y en el tope un paleosuelo bien desarrollado, donde se reconocieron los horizontes 2Bt1, 2Bt2, 2Btk1, 3Btk2, 3BCkm y 4BC. Sobre la unidad II y rellenando grietas que atraviesan la secuencia subyacente, se observaron depósitos de calcretos, discontinuos, de los cuales se obtuvo una datación C14 de 4180 ± 80 a AP. En contacto neto erosivo se dispone la unidad III, franco limo arcillosa, sobre la que se desarrolla el suelo actual con dos horizontes edáficos A y AB. Las secuencias basales de la unidad I se interpretan como depósitos loéssicos o con mínimo retransporte subácueo local, depositados bajo condiciones áridas y frías, correspondientes al Último Máximo Glaciar (ISO2, Pleistoceno Superior). Los niveles aluviales superiores (unidad II), indican incremento en la disponibilidad de agua y ocurrencia esporádica de flujos encauzados, que removilizan localmente los materiales infrayacentes y los depositan en ambientes de baja energía. Esta secuencia representaría la transición hacia condiciones más benignas del período Hypsitermal (Pleistoceno tardío-Holoceno inferior), cuyo clímax se asocia al desarrollo del importante paleosuelo en el techo de la unidad II. Las características pedogenéticas del mismo indican condiciones cálidas y húmedas, y un ambiente de pastizales con alta cobertura, que favorecieron la formación y traslocación de arcillas y óxidos de Fe en el perfil y una importante acumulación de materia orgánica. La formación de una superficie erosiva en el techo del paleosuelo y la presencia de calcretos en diferentes discontinuidades del perfil, se asocian al ciclo más árido del Holoceno mediosuperior, que culmina con la depositación de materiales loéssicos/loessoides (unidad III). Bajo las condiciones templado-húmedas del clima atlántico actual, se restauró el ambiente de praderas de altura, se estabilizó el paisaje y se inició un nuevo ciclo de pedogénesis que, además de la unidad III, incorpora al perfil edáfico infrayacente.<hr/>The summit planation surfaces present in Sierras Pampeanas correspond to antique erosion surfaces generated on Precambrian-Early Paleozoic crystalline basement rocks, which were faulted and tilted during the Andean (Tertiary) orogeny, and subsequently subjected to new denudative cycles. This paleosurfaces are partially covered by Cenozoic sediments (mainly of a Quaternary age), in general of loessic/loessoid origin, with average thickness of less than a meter, with the exception of some sites where they are over 5 meters. The study of these sequences, called deposits of pampas de altura (summit plains), allows making not only an approach to paleoclimatic and paleoenvironmental conditions that characterized the Sierras Pampeanas during the Quaternary, but also a comparative analysis with the surrounding low plains. There are several studies about the Quaternary record of the lowlands in the vicinity of the study region, from the pioneering work of Bodenbender (1894), Doëring (1907) and Frengüelli (1925), until the most recent contributions of Cantú and Degiovanni (1984), Blarasin and Sánchez (1987), Iriondo (1990a,b, 1994), Cantú (1992), Iriondo and Kröhling (1995), Cioccale (1999), Carignano (1999), Schiavo (2003), Cantú et al. (2004, 2006), Kemp et al. (2004a, 2006), Frechen et al. (2009), Sanabria and Argüello (2009, 2011), Tauber et al. (2012), among others. For the mountain areas, there are still few works, such as those by Montes (1958), González (1960), Manzur (1995), Córdoba et al. (2005), Tauber (2006), Tauber and Goya (2006), Tauber et al. (2008) and Krapovickas and Tauber (2012a,b). The objective of this work was to analyze the Quaternary deposits localized in the summit planation surface of the Sierra de Comechingones, between 32º42´-32º50´S and 64º52´-64º59´W (Fig. 1), to establish a preliminary stratigraphic sequence for the area and to suggest the paleoenvironmental and paleoclimatic conditions during sedimentation. The Comechingones ranges, as part of the Sierras Pampeanas, shows classic asymmetrical transverse profiles (Fig. 2). The western side is a fault scarp of high gradient, whose displacement (variable between 200 and 1000 m) decreases toward the South, while the eastern side is a gentle structural slope, with well defined steps, associated to tectonism and/or differential erosion (Fig. 2, A-A´ and B-B´ profiles). In the mountain area, the dominant lithological types are gneissic-migmatitic rocks (the Monte Guazú Complex), mylonites and ultramylonites (the Guacha Corral Shear Zone), and granitic rocks (the Cerro Áspero batholith). The Quaternary deposits are exposed mainly in the piedmont plains, and, in a lesser extent in the mountain valleys covering the plains developed at the summit areas of the ranges. The landscape of the study area shows three major morphoestructural features: 1) submeridian megablocks of Precambrian-lower Paleozoic, crystaline rocks, 2) a western scarp associated to the Comechingones fault (Gordillo y Lencinas, 1979; Introcaso et al. 1987, Costa, 1996), and 3) a lower eastern structural slope, highly dissected by the drainage network after the Andean movements, which preserve relicts of erosion paleosurfaces (Stelzner, 1885; Rovereto, 1911; Penck, 1914, 1920; Rassmuss, 1916; Schmieder, 1921; Rimann, 1926; González Díaz, 1981; Jordan et al., 1989; Costa et al., 1999; Carignano et al., 1999; Rabassa et al., 2010; Degiovanni y Andreazzini, 2013) in the summit areas (Figs. 1, 3). The study included geological and geomorphological recognition, both by remote sensing and field work. Three representative sedimentologicalpedological profiles were described and interpreted (P1, P2 and P3, Figs. 4, 5), while various samples were collected. Chronology (C14 and OSL), granulometry, mineralogy of the clay fraction and organic matter content determinations were made. In the analyzed succession three main stratigraphic units were recognized (Figs. 5, 7). Unit I is formed by light brown (7,5YR 6/4), silty loam materials, with moderately sorting. This unit is mainly massive or shows a very weak lamination. OSL dating of quartz of a sample from unit I yielded an age of 23,0 ± 2,25 ky (Fig. 5b). By means of a sharp contact there is unit II, composed by clay loam materials of light brown color (10YR 6,5/4) at the base to dark brown color (7,5YR 4/2) towards the top. This unit presents trough cross-laminated beds and cut and fill structures (Figs. 5a-d, 7) associated with small channels (Fig. 5c). The top of unit II shows a well-developed paleosol, where several horizons were recognized: 2Bt1, 2Bt2, 2Btk1, 3Btk2, 3BCkm and 4BC (Fig. 6). The diagnostic laboratory and field characteristics are detailed in Table 1 and the figure 8 represents the X-ray diffractograms of samples from the paleosol. On top of unit II and filling cracks which cut the underlying units I and II, there are discontinuous and slightly laminated calcrete deposits. The thickness of these deposits ranges between 1 and 10 cm, and shows lateral variations in their distribution and preservation. Microscopically the calcrete shows a porphiric distribution, and is formed by a micritic mass that include loessoid material (Fig. 9). A radiocarbon age 4.180 ± 80 yr BP was determined in this calcrete. The calibration informed by the laboratory was performed using the CALIB 6.0.1 program, in conjunction with the recommendations of Stuiver and Reimer (1993). According to the calibration curve, the event of carbonation is limited between 4.529 and 4.814 years AP. Above unit II, or above the calcrete level when it is present, and through an erosional sharp contact there is unit III. It is a silty clay loam, with dark gray (10YR 4/1) and dark grayish brown (10YR 4/2) color and 0.3 to 0.4 m of thickness (Figs. 5, 7). These sediments are partially modified by pedogenesis, showing two horizons, A and AB (Fig. 6, Table 1). Unit I is interpreted as a loessic deposit, undisturbed or with minimum local reworking, sedimented under arid and cold conditions. According to the obtained OSL age the loess was accumulated during the late Pleistocene, then corresponding to the Last Glacial Maximum (ISO2). The above alluvial levels of unit II indicate higher water availability and sporadic occurrence of torrential erosive channeled flows, which locally removed the underlying loessic materials and deposited them in subaqueous low energy environments. Unit II would represent the transition to more benign conditions of the Hypsithermal period (Late Pleistocene-Early Holocene), whose climax is here linked to the recognized paleosol on top of unit II. The pedogenetic characteristics of this paleosol would indicate warm and wet conditions (higher than at present), and an environment of meadows with grassland of high cover. This condition favored both formation and traslocation of clay and iron oxides in the soil profile, and the accumulation of high contents of organic material, all features observed in the paleosol. The formation of an erosive surface with decapitation of the paleosol, and the presence of calcretes on different profile discontinuities (textural, desiccation cracks), are associated to the Mid-Late Holocene more arid cycle, that ends with the deposition of the aeolian loessic material of unit III. Under the present temperate-wet Atlantic climatic conditions, the environment of tall grasslands was restored, the landscape stabilized, and a new pedogenesis cycle started which affected unit III and the underlying paleosol. <![CDATA[Eolian deposits associated with the alluvial plain of the Desaguadero River, Mendoza province, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792013000200004&lng=en&nrm=iso&tlng=en Asociado a la margen derecha del río Desaguadero, colector principal de los cursos fluviales que drenan el piedemonte de la región andina central, y la zona de interacción con el piedemonte distal mendocino, se disponen destacadas acumulaciones arenosas eólicas que han recibido escasa atención hasta la fecha. Durante investigaciones recientes se ha determinado que estos depósitos se pueden diferenciar en relación a una variedad de geoformas eólicas tales como dunas lunetas, dunas longitudinales y mantos arenosos. En conjunto, estas unidades eólicas se disponen estratigráficamente sobre la Fm. Arco del Desaguadero, sucesión fluvio-lacustre desarrollada entre el Pleistoceno superior y el Holoceno superior. Los depósitos eólicos más antiguos forman dunas lunetas de hasta 14 m de altura, y se los asocia a la deflación de arenas desde las costas occidentales de lagunas temporales vinculadas a inundaciones del río Desaguadero. En base a dataciones numéricas realizadas sobre restos de conchillas de gasterópodos, se asignan estas dunas al Optimo Cálido Medieval. Posteriormente, durante el deterioro climático vinculado a la Pequeña Edad de Hielo, se desarrollaron dunas longitudinales y mantos arenosos con aportes sedimentarios provenientes de la llanura aluvial del río Desaguadero y de la removilización de arenas de las dunas lunetas previamente formadas. Se distinguen además dunas lunetas de menor envergadura asociadas a depresiones de deflación y encharcamiento temporal y extensos mantos arenosos de escaso espesor (menores en general a los 50 cm), generados por sedimentación eólica y posterior removilización por la escorrentía superficial, cuyo desarrollo se estima habrían comenzado también durante este último periodo. La disposición espacial de las dunas lunetas y las dunas longitudinales permiten establecer paleovientos provenientes del Este y Sureste.<hr/>On the right bank of the Desaguadero river and the distal piedmont of the Andean Precordillera, there are important aeolian accumulations that have received little attention to date (Figs. 1,4). These deposits, recognized in different previous regional studies, are grouped within a complex unit called "Médanos y Guadales" (Polanski, 1963; González Díaz y Fauqué, 1993; Rodríguez y Barton, 1993; Costa et al., 2001). The main objective of this work was to analyze the geomorphological characteristics of the area in detail, considering sedimentological, stratigraphic and chronological features of the units, in order to make a contribution into the knowledge to date about the regional geological evolution. The geomorphological analysis was based on satellite image interpretation by means of a Geographic Information System (GIS). Fieldwork was conducted to verify the characteristics of mapped units and get further details on their morphological and morphometric traits. A general sampling of the units was made taking superficial sediment through small holes up to 50 cm - 1 m deep. Sediment samples were processed at the laboratory for particle size characterization by sieving and Bouyoucos techniques, and mineralogical composition by binocular magnifying glass (10X) and microscope. Conventional radiocarbon dates were obtained based on residues gastropod shells, calcium carbonate and carbon. The geomorphological analysis allowed to establish that aeolian deposit consists of three major landforms as lunette dunes, linear or longitudinal dunes and sandy mantles (Fig. 6). The lunette dunes are landforms of particular importance in the region because of the information they provide as a paleoenvironmental and paleoclimatic indicator. The size of this type of dune, with asymmetrical cross sections and face steeper windward, is influenced by the size of the beach or ephemeral lake which is associated to it (Thomas, 1989; Holliday, 1997; Lancaster, 2005). Pye and Tsoar (2009) note that as parabolic dunes, the arcuate plan form of the lunettes indicates wind direction, and the sedimentation on the surface of the dune is generally improved by the presence of vegetation. They further note that, unlike the parabolic dunes, lunettes are rarely transgressive causing fix dunes. In this study we differentiated lunette dunes associated with temporary ponds (DL1) and lunette dunes associated to deflationary depressions and temporary ponding (DL2). The former develop large parallel cords to morphology of coastal temporary ponds and can reach several kilometers long and up to 14 m high. They are distributed in at least five altimetric levels which are linked to different stages in an overall shrinkage of river flows during the Holocene (Figs. 7,8). The DL2 correspond to lunette dunes of smaller dimensions, with less than 3 m heights and lengths between 300 and 700 m. They also shows a characteristic plan morphology with a straight central section and ends in "horns" pointing eastward. The longitudinal sand dunes occur commonly grouped into dune fields, with variable wave lengths between 100 and 200 m, 2 to 4 m height and 100 to 400 m long, and sandy mantles are characterized by their large surfaces and thin thicknesses, generally less than 50 cm. The superficial texture of aeolian deposits shows a difference between the lunette dunes DL1, with predominantly sandy texture, regarding DL2, with sandy loam textures, which can be associated with higher energy, fluvial-lacustrine depositional environment of the source areas. Longitudinal dunes have a higher content of medium sized sand than lunettes dunes, possibly associated with higher wind energy. The sandy mantles have higher contents of silt than longitudinal dunes and lunette dunes, due to its reworked by alluvial action. The mineralogical composition indicates a general predominance of quartz and subordinate plagioclase, with abundant undifferentiated lithic fragments. The gypsum and fragments of shells are abundant in the lunettes dunes, with presence of shattered glass that usually does not exceed 5% of the total sample. These data are consistent with the mineralogy of the fluvial deposit of the Desaguadero riverbed, from where it is estimated to come most of the grains. The spatial arrangement of the lunette dunes allows inferring winds from East and Southeast. The lunette dunes comprise the oldest preserved aeolian deposits in the study area and rest on the Arco del Desaguadero Formation, whose period of deposition comprising the upper Pleistocene-Holocene. Based on the dates obtained in the Alto Grande lunette dune (1210 ± 50 C14 yr BP and 690 ± 90 C14 yr BP) its formation is assigned to upper Holocene, during the period known as the Optimum Medieval Warm when climatic conditions have allowed further development of the vegetation cover and the fixing of temporary ponds marginal cords. The geomorphological and stratigraphic feature of the longitudinal sanddunes allow to infer subsequent development of lunette dunes during the Little Ice Age by means of winds coming from East and Southeast. The development of sandy mantle is considered partially contemporaneous with the development of longitudinal dunes with posterior remobilization associated with sporadic surface runoff. Currently the aeolian morphogenesis is partially restricted due to a higher density of vegetation cover, which is linked to the general increase in precipitation, at least during the last 60 years. <![CDATA[New microscopic evidences of the autochthony of the ferrallitic pedological mantle in the Misiones province, Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792013000200005&lng=en&nrm=iso&tlng=en The Misiones province is characterized by a pedological mantle with Ultisols, Oxisols and Alfisols. This ferrallitic clayey material frequently shows one or more siliceous or ferruginous nodular "stony horizons" close to the saprolitized basalt, which are generally associated with "structured horizons". The autochthonous or allochthonous origin of the materials composing these complex profiles has been controversially discussed. In this work, we present the results of new microscopic analyses of thin sections and of sand and silt fractions from several profiles of this pedological mantle. The results show that the "nodular horizons" are formed in the massive basalts by weathering and segregation of saprolitized fragments, which become progressively rounded, ferruginized and indurated. Concurrently, the groundmass in- between the nodules is enriched in clay and iron oxides, developing blocky peds, which gives rise to the "structured horizons". On the other hand, the vesicular porosity fossilized by chalcedony in the solum of a profile demonstrates the residual origin of the soil parent material, developed by the weathering of a vesicular-amygdaloidal basalt bearing hydrothermal quartz veins. In turn, the morphology and the surface texture of quartz grains from the sand and silt fractions analyzed by optical microscopy and by scanning electron microscopy reveal processes of silica dissolution and precipitation and show no signals of eolian transport. In accordance with previous studies, these microscopic results provide clear evidence that the parent material of soils in the Misiones province and the "stony horizons" usually included in these profiles are the result of in situ weathering of the diverse basaltic flows outcropping in the region.<hr/>La provincia de Misiones está caracterizada por un manto pedológico en el que predominan Ultisoles, Oxisoles y Alfisoles. Este material ferralítico arcilloso presenta frecuentemente cerca del basalto saprolitizado uno o más horizontes o "líneas de piedra" constituidos por cuarzo o por nódulos ferruginosos, los que están en general asociados a "horizontes estructurados". El origen autóctono o alóctono de los materiales que constituyen estos perfiles complejos ha sido objeto de controversias en la literatura. En este trabajo se presentan los resultados de nuevos análisis microscópicos de cortes delgados y de las fracciones arena y limo de diversos perfiles de este manto pedológico. Los resultados obtenidos muestran que los "horizontes nodulares" se forman en los basaltos masivos por meteorización y segregación de fragmentos saprolitizados, los que adquieren forma esferoidal y devienen progresivamente ferruginizados e indurados; paralelamente la masa basal entre nódulos se enriquece en arcilla y óxidos de hierro desarrollando una estructura en bloques que da lugar a los "horizontes estructurados". Por otro lado, la porosidad vesicular fosilizada por calcedonia en el solum de un perfil, demostró el origen residual del material parental del suelo formado a partir de la meteorización de un basalto amigdaloide-vesicular que incluye venas de cuarzo hidrotermal. Además, los análisis morfoscópicos y exoscópicos de los granos de cuarzo en las fracciones limo y arena mediante microscopía óptica y electrónica revelan procesos de disolución y precipitación de sílice y no muestran evidencias de transporte eólico. En consecuencia, y en coincidencia con estudios previos, estos resultados microscópicos confirman que el material parental de los suelos de la provincia de Misiones y los "horizontes pedregosos" usualmente incluidos en los perfiles son el resultado de la meteorización in situ de las diversas coladas basálticas aflorantes en la región.