Scielo RSS <![CDATA[Latin American journal of sedimentology and basin analysis]]> http://www.scielo.org.ar/rss.php?pid=1851-497920100002&lang=en vol. 17 num. 2 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <![CDATA[Estratigrafía de las rocas metasedimentarias (Neoproterozoico-Cámbrico) de la Sierra de Mojotoro, Cordillera Oriental Argentina]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792010000200001&lng=en&nrm=iso&tlng=en El presente trabajo consiste en un estudio geológico detallado de las metamorfitas (Fm Puncoviscana s.l. o Complejo Puncoviscana) que constituyen el núcleo estratigráfico de la sierra de Mojotoro, Cordillera Oriental Argentina. El análisis integrado de este conjunto de rocas permitió dividir la clásica Fm Puncoviscana s.l. o Complejo Puncoviscana en tres unidades estratigráficas, que de más antigua a joven son: las Formaciones Chachapoyas, Alto de la Sierra y Guachos. La Formación Chachapoyas aflora en el flanco occidental de la sierra; está compuesta por facies de metapelitas y metaareniscas verdosas de grano muy fino, con un clivaje dominante de plano axial. Las rocas están fuertemente plegadas y afectadas por un metamorfismo dentro del campo de la anquizona alta-epizona. La Formación Chachapoyas esta intruida por diques ácidos de una edad de 533 ± 2 Ma. La facies arenosas son del tipo cuarzosas de grano fino clasto-soportadas y las facies pelíticas están compuestas por illita y clorita. La Formación Alto de la Sierra es una unidad metaarenosa, compuesta por metavaques cuarzosas, líticas y feldespáticas, donde la característica más notable es la presencia de material volcaniclástico, claramente identificable a través de corte delgado. Las arcillas de esta unidad son del tipo illita y escasos interestratificados de illita/esmectita. Las rocas fueron afectadas en un grado metamórfico tipo anquizona débil a intensa. La Formación Guachos constituye una secuencia heterolítica de metaarenisca-metapelita de coloraciones parda, pardo verdosas, en la cual se identificaron trazas fósiles de la icnofacies de Nereites. Las metaareniscas son del tipo cuarzosas donde se reconocen algunos líticos volcánicos de textura felsítica y las metapelitas están compuestas por illita, illita/clorita y clorita. Las rocas se encuentran plegadas y afectadas por un evento de deformación reconocible a escala de campo y en corte delgado. El grado de metamorfismo al cual estuvieron sujetas las rocas es dentro del campo de la anquizona intensa-epizona.<hr/>The basement of Argentine Eastern Cordillera is an heterogeneous succession of siliciclastic and chemical rocks of Neoproterozoic-Early Cambrian age, identified as the "Precambrian basement" (Keidel, 1910) underlying with angular unconformity the Cambrian and Ordovician deposits of the Mesón and Santa Victoria Groups. The unit was defined as the Puncoviscana Formation (Turner, 1960) in the Santa Victoria range, and assigned to the Neoproterozoic-Cambrian (Mirré and Aceñolaza, 1972) due to the presence of Oldhamia, a trace fossil of the Lower Cambrian. However, the Puncoviscana Formation s.l. (Aceñolaza and Aceñolaza, 2005) is often adopted to identify all the basement of Eastern Cordillera, representing a heterogeneous group of rocks affected by low-grade metamorphism. The basement of the Mojotoro range is part of the 1a tecto-metamorphic zone, as defined by Willner (1990). They are part of a near-surface structural level in which the sedimentary characteristics and the original microfabric could be studied in detail (Fig. 1). From structural, stratigraphic and mineralogical information, a discontinuity (fault) could be inferred, and a new Early Cambrian stratigraphic unit named the Guachos Formation (Moya, 1998) in the Mojotoro Range can be defined. The main purpose of this contribution is to characterize the similarities and differences between the stratigraphic units of the basement in the Mojotoro Range (Lower-Middle Cambrian) in order to assess their relative stratigraphic position. The metamorphic grade was established from the cartographic material and sedimentological, petrographical and mineralogical characterization, and enabled the identification of three stratigraphic units (Fig. 1): the Chachapoyas, the Alto de la Sierra and the Guachos Formations. Satellite images and aerial photographs were used to build up a map of the studied area, with structures, lithologic contacts and sampling sites placed using GPS. The compositional studies of the field samples were carried out with a binocular magnifying lens, optic and scanning electronic microscope (SEM), and X-ray diffraction (XRD). The Chachapoyas Formation (nomen. novum.) crops out in the western flank of the range (Fig. 1), and consists of shales and very fine-grained, greenish sands, with a cleavage parallel to the axial plain (Fig. 3e). Rocks are strongly folded and affected by high anchizone-epizone grade metamorphism (Fig. 6). The unit is intruded by acidic dikes, dated at 533 ± 2 Ma, showing that the Chachapoyas Formation age is older than the other units in the Mojotoro basement (Fig. 2). Sandy facies are thin and mainly composed of grain-supported quartz arenites. Clay mineralogy corresponds to illite and clorite. Another described unit is the Alto de la Sierra Formation (nomen. novum.), composed by layers of purple and gray sands with parallel bedding, shales and feldespatic metawackes (Fig. 3a-c). Volcaniclastic material is recognized in thin sections (Fig. 4a-d), and clays mineralogy corresponds to illite and interlayer illite/smectite (Fig. 7c). The unit was affected by a weak to intense anchizone metamorphic grade (Fig. 6). A heterolitic sequence of brown to greenish brown shaly sands, with Nereites ichnofacies (Fig. 3f-h), was recognized as the Guachos Formation (Moya, 1998). The sandstones are mainly composed of quartz, and less proportions of volcaniclastic grains with felsitic texture (Fig. 3f-h). Shales are composed of illite, illite/chlorite and chlorite (Fig. 5b, d). The outcrops are folded and affected by a deformation event, a characteristic reflected in thin sections. The metamorphic grade corresponds to a high anchizone-epizone (Fig. 5a). The Guachos Formation can be correlated to the volcaniclastic rocks of Rancagua area, which presents similar facies and Nereites ichnofauna. The Rancagua succession has been studied by different authors (Lork et al., 1990; Adams et al., 2008) that described several rhyolitic detrital zircons in this unit and obtained a maximum age of sedimentation of Lower-Middle Cambrian. Finally, the fault between the Guachos Formation and the Chachapoyas Formation (Fig. 1) corresponds to a possible Neoproterozoic-Cambrian unconformity, and the acidic dikes intruded in the Chachapoyas Formation could correspond to a part of the calc-alkaline Pampean magmatic arc, defined in the Eastern Pampean ranges, with a great extension in NW Argentina, were the volcanic cover was eroded and could represent the source rock of the Alto de la Sierra and the Guachos Formations. <![CDATA[Paleoambientes de la cuenca media del río Luján (Buenos Aires, Argentina) durante el último período glacial (EIO 4-2)]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792010000200002&lng=en&nrm=iso&tlng=en El objetivo de este trabajo es definir unidades depositacionales y realizar inferencias paleoambientales a partir del análisis de facies y del contenido paleobiológico (moluscos, fitolitos y diatomeas) de depósitos acotados cronológicamente entre ca. <70.000 años y 11.000 años AP., Estadios Isotópicos de Oxígeno (EIO) 4 a 2, para la cuenca media del río Luján. Se seleccionaron tres secciones en las cuales se realizaron un total de siete perfiles estratigráficos de detalle y donde se extrajeron las muestras para los análisis sedimentológico, del contenido paleobiológico y para dataciones. Los depósitos estudiados fueron agrupados en cinco facies sedimentarias (F1, F2, F3, F4 y F5), ubicados cronológicamente y correlacionados con unidades semejantes descriptas por otros autores. Se reconocieron varias discontinuidades erosivas y de estabilidad para el lapso Pleistoceno tardío-Holoceno, las que permitieron definir cinco unidades depositacionales (UD). La presencia en el río Luján de una barranca, donde se exponen depósitos de edad Pleistoceno tardío-Holoceno, está estrechamente asociada a la incisión de los depósitos de relleno de antiguas depresiones interconectadas, ocurrida durante el Holoceno. Esto permitió inferir que la paleotopografía de esta área ha ejercido un control determinante en la existencia de barrancas con afloramientos del Pleistoceno tardío-Holoceno. Aún cuando las sucesiones son incompletas y las dataciones no permiten ajustar el inicio y la finalización de las UD para el lapso EIO 4 a 2, el análisis facial sugiere la existencia de una marcada variabilidad ambiental durante ese lapso, para el noreste de la provincia de Buenos Aires. En función del análisis facial se interpretó que el ambiente de depositación correspondió a sistemas fluvio-lacustres efímeros y eólicos, con sedimentación para el lapso ca. <70.000 años - <11.000 14C años AP, bajo condiciones áridas, principalmente climas seco-subhúmedo y semiárido. Durante al menos una parte del EIO4 y durante el EIO2 habría ocurrido depositación eólica predominante, durante intervalos climáticos sub-húmedos secos. En tanto durante el EIO3 habrían tenido lugar eventos aluvionales episódicos bajo condiciones climáticas subhúmedas a subhúmedas secas o fuertemente estacionales y con alternancia de fases sub-húmedas-húmedas. Esto indicaría que las acumulaciones habrían tenido lugar mayormente en ambientes depositacionales con déficit hídrico aunque de variado grado. Las características geomorfológicas y las evidencias sedimentológicas y paleobiológicas permitieron inferir que el clima fue el forzante principal en el desarrollo de la sucesión sedimentaria.<hr/>The sedimentary sequences exposed in cutbanks of the middle course of the Luján river preserve important paleoenvironmental information on the late Quaternary climatic evolution of the north-eastern Pampean region. The objective of this paper is to define depositional units and infer the paleoenvironmental conditions from the analysis of the sedimentary facies and the paleobiological content (mollusks, phytoliths and diatoms) of the sequences dated between ca. 70,000 and 11,000 yr BP, Oxygen Isotopic States (OIS) 4 to 2, of the middle region of the Luján river basin. This paper follows on from previous studies about the reconstructions of the paleoenvironmental and climatic changes in the northern region of the Buenos Aires Province presented by Dangavs and Blasi (1995), Prieto et al. (2004) Fucks et al. (2005), Fucks and Deschamps (2008) and Blasi et al. (2008, 2009a, b). A late Pleistocene-Holocene cutbank (PT-H) is preserved along the banks of the Luján river where three sections were analyzed (Table 1). The lower exposures, found from the present river level to halfway up the bank show late Pleistocene materials, while the upper portion shows Holocene fluvio-lacustrine and aeolian deposits. The Holocene deposits overlie paraconformably and start with a high organic matter concentration level which was deposited ca. 11,000 14C yr BP in a lentic environment of meso-eutrophical characteristic (Prieto et al., 2004). Between the Jáuregui and Manzanares cities the PT-H cut bank is not continuous but alternates with deposits from a previous sedimentary cycle that we informally named "Pampeano" (Ameghino, 1884; Dangavs and Blasi, 1995). This last unit constitutes the present fluvial channel floor and interfluvial deposits of this region. The development of the cutbanks studied here is likely to be related to a late Holocene fluvial incision of the late Pleistocene-Holocene sediments and aeolian, fluvial and lacustrine deposits that infill blowout depressions (Ameghino, 1880-1881, 1884; Frenguelli, 1925; Dangavs and Blasi, 1995). Three sections along the middle course of the Luján river were selected for the production of seven detailed stratigraphic profiles (Table 1), for collecting samples for sedimentological and paleobiological analyses (Table 2), and for radiocarbon and IRSL dating (Table 3). The late Pleistocene sedimentary record (ca. <70,000 - <11,000 14C yr BP) between the localities of Jáuregui and Manzanares allowed recognition of five facies: F1, F2, F3, F4 and F5 (Table 2), which are either totally or partially present in the studied sequences. The facies were correlated to the units described by other authors (Table 4). The studied sequence starts with silty gravel and gravelly silt of lacustrine and fluvial nature (facies F1), that grade upward or laterally to sandy silt to silty sand of facies F2 that represent dunes of pellets (Figs. 4 and 5). Facies F2 presents an irregular based and low angle cross-bedding, and is composed of fine sedimentary lithoclasts, quartz and feldspar grains. The biological indicators are represented almost exclusively by phytoliths. This facies would have been produced because of seasonal wind storms that deflated sediments of the marginal platform of the lentic bodies that had been exposed by the retraction of the water body during temperate sub-humid dry to seasonal conditions. Facies F1 and F2 are unconformably covered by sandy gravel, gravelly sand and sand within a sandy silt matrix, included in the facies F3. It forms a massive lenticular stratum with commonly carbonatic clasts ("tosca"), reworked fragments of bones, siliciclastic grains and fine sedimentary intraclasts (Figs. 4-6). Remains of ostracods, diatoms, extinct mollusks and phytoliths are also abundant in facies F3 (Table 5). These deposits were accumulated after successive ephemeral fluvial episodes, because of heavy and concentrated rains of different frequency and caudal from channeled flow in gullies. During each fluvial event, temporary lentic water bodies would have originated because of either damming or overflow, thus producing the accumulation of fine material which, in part, was infiltrated as the matrix of the coarse deposits. The paleobiological records, frequently monospecific, suggest conditions of environmental stress due to abrupt salinity changes, caudal and temperature. These environments were very changeable because of water overflow produced during storms, and with low water levels after high evaporation periods. During facies F3 deposition, distal and local aeolian inputs would have continuously taken place with low sedimentation rates and without aeolian landforms; instead the aeolian materials would have been amalgamated to the alluvial ones. Sedimentation under temperate to cold conditions with alternant sub-humid/humid facies and dry or highly seasonal facies is interpreted here. Subsequently, the sediments included in facies F4 are vitroclastic sandy silt and silt (Figs. 4-6) with higher abundance and diversity of diatoms and phytoliths of Poaceae, Cyperaceae and woody dicots. This facies would correspond to loess deposits that accumulated in flooded lowlands, lentic shallow water bodies, and under temperate to cold and sub-humid to highly seasonal conditions. The succession ends with sandy silt of the facies F5 (Fig. 6), with very scarce biological content. These deposits represent the degradation of the lentic water bodies due to clastic infilling under temperate sub-humid/dry conditions that formed alkaline swamps. The depositional paleoenvironments for the lapse ca. <70,000 - <11,000 14C yr BP were interpreted as ephemeral lake-fluvial and aeolian settings in dry-subhumid and semiarid basins. Three major erosion discontinuities were recognized (Fig. 8). The first discontinuity would have occurred during late Pleistocene times, the second one corresponds to the late Pleistocene-Holocene transition and the third discontinuity would have taken place during historical times. Two other discontinuities were observed and are marked by indicators of stability like the presence of a duricrust on the top of the facies F3 and a paleosoil on the top of the late Holocene lake-fluvial deposits (Prieto et al., 2004). These discontinuities allowed recognition of five depositional units (DU) or aggradation events in the middle basin of the Luján river within the late Pleistocene-Holocene sub-cycle (Zárate, 2005). The DU 1 consists of facies F1 and F2, with ephemeral lacustrine-fluvial and aeolian deposits, respectively. It represents sedimentation in lowlands interconnected by ephemeral streams, which were active during instantaneous current episodes, derived from storms, with the generation of ephemeral lakes and formation of aeolian lunettes towards the end of the flooding cycle. The minimal depositational age of DU 1 was inferred as being ca. 60,000 yr BP. The DU 2 includes the facies F3 of ephemeral fluvial to ephemeral lacustrine deposits (the later of eutrophic to distrophic characteristics). This sedimentation took place in lowlands interconnected by ephemeral streams during instantaneous current episodes derived from storms and the formation of temporary lakes. Different from the previous DU 1, this multiepisodic accumulation was produced under cold, dry conditions. The inferred depositional lapse was estimated to be between ca. 50,000 yr BP and 32,500 yr BP. The DU 3 is formed by facies F4 and F5. Its accumulation started with settling in permanent lakes or ponds with variable inputs of aeolian sand and dust, due to wind storms, under temperate and sub-humid climatic conditions (facies F4). Subsequently, these lentic water bodies were degraded by dystrophy (facies F5). The temporal interval of DU 3 accumulation was dated between 37,700 yr BP and 11,000 yr BP. The DU 4 includes lake-fluvial to marsh deposits of eutrophic to distrophic characteristics, analyzed by Prieto et al. (2004). According to these authors this deposition took place in lowlands interconnected by permanent streams, under humid to sub-humid/dry conditions after 7,000 yr BP. The deposition lapse was dated between ca. 11,000 and 3000 yr BP. The DU 5 consists of sandy-silty facies corresponding to reworked aeolian deposits that buried a paleosoil of ca. 3,000 yr BP (Prieto et al., 2004). They interpreted this unit as deposited under climatic conditions similar to the present ones since 3000 yr BP and until the incision of the present channel in historical times. Two aeolian episodes of large magnitude were recognized, presented in the facies F2 (=56,400 ± 6,500 yr BP) and facies F4-F5 (=32,500 ± 4,700 to =11,000 yr BP). The deposition of distal or local aeolian material could have been continuous, but, during some stages of lower deposition rate, aeolian deposits might have been integrated in the alluvial deposits. During the late Pleistocene, the seasonal regime of these environments would have influenced the aeolian and ephemeral-fluvial deposition processes, with differing degrees of participation in the sedimentation. The present day middle reaches of Luján river may have constituted, at that time, the head of the basin with low order ephemeral or temporary streams, so the influence eustacy is considered negligible (Blasi et al., 2009a). The succession studied here shows that the environmental variability during the OIS 4 to 2 in the northern Buenos Aires province would have been higher than previously interpreted. Episodic alluvial events probably took place under sub-humid climatic conditions during the OIS3, as shown by the paleobiologic records, and aeolian deposition was predominant during most of the OSI2 and, at least, part of the OIS4, evidencing sub-humid/dry climatic conditions. Sedimentological and paleobiological records of the middle course of Luján river allowed the inference that climate was the main driving force for sedimentation. <![CDATA[Análisis del contacto entre las formaciones Vinchina y Toro Negro (Sierra de los Colorados, provincia de La Rioja, Argentina), sus implicancias tectónicas]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792010000200003&lng=en&nrm=iso&tlng=en En este trabajo se analiza la expresión estratigráfica y significado geológico de la superficie que separa a las formaciones Vinchina (Mioceno) y Toro Negro (Mioceno Tardío-Plioceno), las que afloran conformando la Sierra de Los Colorados (oeste de la provincia de La Rioja). El estudio se realizó mediante el levantamiento de 6 secciones detalladas, de norte a sur: quebrada de La Aguada, quebrada de Los Pozuelos, río de La Troya norte, río de La Troya sur, Campo Negro y quebrada del Yeso. La información reunida en estos perfiles permitió comprobar que el contacto entre ambas unidades queda definido por una superficie de incisión de alto relieve relativo que suprime, en algunos sectores, hasta un 25% de la Formación Vinchina. La superficie en cuestión forma un paleovalle en el sector norte (secciones de La Aguada y Los Pozuelos), se manifiesta como una superficie de incisión ondulada (con mínima supresión estratigráfica) en la parte central del área estudiada (secciones de río de La Troya sur y Campo Negro) y pierde relieve hacia el sur hasta transformarse en una superficie relativamente plana (sección de quebrada del Yeso). El análisis de los depósitos sobrepuestos a la superficie de incisión permitió la definición de cuatro secciones estratigráficas. La sección S1 está compuesta por conglomerados extraformacionales, aglomerados y brechas intraformacionales. En la sección S2 predominan conglomerados gruesos hasta finos, areniscas guijarrosas y areniscas gruesas siendo muy escasas las pelitas y areniscas muy finas. La sección S3 comprende ciclos granocrecientes formados por pelitas, areniscas y conglomerados. Finalmente, la sección S4 comprende pelitas y areniscas finas con limitada participación de conglomerados y areniscas gruesas. La sección S1 es interpretada como depósitos confinados bajo condiciones de bajo espacio de acomodación. La sección S2 también fue formada en condiciones de confinamiento pero dentro de un esquema de espacio de acomodación creciente. Finalmente, las secciones S3 y S4 indican la pérdida de confinamiento del sistema y el estado de máximo espacio de acomodación. La génesis de la superficie de incisión que separa a las formaciones Vinchina y Toro Negro es, en este trabajo, relacionada a las fases principales del ascenso del Famatina occidental.<hr/>The stratigraphic expression and geological significance of the contact between the Vinchina (Late Miocene) and Toro Negro (Late Miocene-Pliocene) formations (Turner, 1964; Ciccioli et al., 2010) is analyzed in this paper. These units with more than 8,000 m of sediments (Ramos, 1970; Tripaldi et al., 2001; Ciccioli, 2008) represent the main units of the infill of the Vinchina Basin (Ciccioli et al., 2010) (Figs. 1-3) during the transition from a simple foreland basin stage to a broken foreland one (Ciccioli, 2008; Ciccioli, et al., in press). They mostly represent sedimentation by fluvial and aeolian processes in an inland basin during an overall warm and dry climate period (Tripaldi et al., 2001; Ciccioli, 2008). Thus, major changes in sedimentation are though to correspond with tectonic events (Ciccioli et al., in press). The lower member of the Vinchina Formation and the upper member of the Toro Negro Formation present a more uniform and extended sedimentation pattern. In contrast, the upper member of the Vinchina Formation and the lower member of the overlying Toro Negro Formation show important lateral (north-south) facies changes (Fig. 4). The along-strike differences in the character of the discontinuity marking the boundary between the Vinchina and Toro Negro formations, is the most meaningful of them all. Six sections, from north to south (La Aguada creek, Los Pozuelos creek, north La Troya river, south La Troya river, Campo Negro and del Yeso creek) were measured across the boundary between the Vinchina and Toro Negro formations along the Sierra de Los Colorados (Fig. 2). Facies associations within this interval were defined and sedimentary paleoenvironments interpreted. The information gathered from these sections allowed interpreting the contact between the Vinchina and Toro Negro formations as a high-relief incision surface that in some places suppresses up to 25% of the underlying Vinchina Formation (Figs. 4 and 5). This surface forms a west-east oriented paleovalley in the north (La Aguada and Los Pozuelos sections), is a low-relief incision surface (showing minimum stratigraphic suppression) in the central part of the study area (south La Troya river and Campo Negro sections) and becomes in a non-erosive planar surface at the southern end (del Yeso creek, Figs. 6 and 7). Considering the available radiometric data (Ciccioli et al., 2010), the age of the unconformity can be bracketed between 19.1 and 8.6 Ma. Four stratigraphic sections were defined in the deposits of the Toro Negro Formation overlying the incision surface (Table 2). Section S1 is composed of extraformational conglomerates, agglomerates and intraformational breccias. Coarse-grained conglomerates, gravelly sandstones and coarsegrained sandstones predominate in section S2 together with scarce mudstones and very fine-grained sandstones. Section S3 comprises coarsening-upward sequences composed of mudstones, sandstones and conglomerates. Finally section S4 is made up by mudstones and fine-grained sandstones with scarce levels of conglomerates and coarse-grained sandstones. Section S1 is interpreted as incision-confined fluvial deposits accumulated under low-accommodation conditions. Section S2 is also interpreted as confined to the paleovalley but with increasing accommodation space. Finally sections S3 and S4 mark the shift from confined to unconfined conditions under high accommodation. The Vinchina Formation was reported to crop out in the higher areas of the Famatina range but up to now there is no record of the Toro Negro Formation in that area. The deep, long-lived, west-east oriented paleovalley described in the northern part of the area is an atypical feature for the foreland basin model. However, the lack of a relationship with contemporaneous marine deposits and evidences of an arid climate throughout the deposition of the Vinchina and Toro Negro Formations suggest a tectonic control for the changes in the erosion vs accumulation rates in the basin. Therefore the incision and infill of the paleovalley is interpreted using the relationship between the fluvial equilibrium-profile and the channel profile (C.f. Dalrymple et al., 1998; Shanley and McCabe, 1994; Blum and Törnqvist, 2000) (Fig. 8) The meandering fluvial system interpreted from deposits of Facies Association (FA) V of the Vinchina Formation (Tripaldi et al., 2001) represents a stage of high accommodation (point A in Fig. 8). After that stage, a progressive lowering of the relative equilibrium profile reduces the accommodation the development of amalgamated channel belts of the fluvial systems interpreted for FA VII (Tripaldi et al., 2001) similar to the "unconfined stream equilibrium profile low" of Dalrymple et al. (1998).This trend continued until the equilibrium profile places below the channel profile. At that point erosion and sediment bypass took place. The major degree on incision and sediment bypass occurred at point C in figure 8 but erosion and down-cutting occurs until point E. At this latter point the equilibrium profile passes above the stream profile and aggradation begins within the incised valley ("confined stream equilibrium profile low" de Dalrymple et al., 1998). Finally, maximum accommodation (point F in figure 8) is represented by the anastomosing fluvial system with encased channels in fine-grained overbank deposits of section S4. As neither the west-east orientation of the observed paleovalley nor the evolution of the sedimentary environments can be explained by the eastward advance of the fold and thrust belt in a simple foreland basin we interpret that the uplift of the Famatina range (cf. Ramos et al., 2002; Davila and Astini, 2007; Davila, 2010) to the east of the studied area (Fig. 9) might have produced a narrow area of high subsidence modifying the stream profiles of the rivers draining from the Andes. In the distal area (Los Colorados range) stream profile dropped below the equilibrium profile causing deep incision while close to the uplifted range rapid deposition might have occurred. Therefore the incision surface separating Vinchina and Toro Negro Formations is related to the main phases of uplift of the western Famatina range (Figs. 10 y 11). This event produced accelerated subsidence and increased accommodation in the proximal area and lowering of the equilibrium fluvial profiles (and incision) in the distal areas to the west (Fig. 10b). During post-tectonic times, subsidence and accommodation rates decrease close to the uplifted area and the relative rise of the fluvial equilibrium profiles in the distal area with renewed aggradation (Fig. 10c). A model explaining the interpreted changes in accommodation relative to the subsidence due to the uplift of the Famatina is shown in figure 11.