Paleoambientes de la cuenca media del río Luján (Buenos Aires, Argentina) durante el último período glacial (EIO 4-2)

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.<br>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 -