The microfauna assemblages as indicators of paleoenvironmental changes in the Miocene fluvial-lacustrine cycles (NE Duero Basin, Spain)

espanolLos depositos siliciclasticos y carbonatados se encuentran intercalados en el area de Villadiego (Mioceno, NE cuenca del Duero). Se han subdividido en dos secuencias de deposito de alto rango: DDS y CDS. El analisis de estas unidades sedimentarias y el estudio del contenido de microfauna, principalmente ostracodos, condujo a la identificacion de sistemas de interaccion lacustre-fluvial. Las caracteristicas sedimentarias revelan la existencia de los sistemas fluviales de grava, llanuras de inundacion y sistemas lacustres que estaban interconectados e intimamente relacionados en direccion N-S. En el analisis sedimentologico se reconocieron trece tipos de litofacies fluviales y lacustres y seis asociaciones de facies geneticas. La parte superior del DDS es el resultado de levantamientos del nivel del lago. El CDS muestra un ciclo de profundizacion-somerizacion. Los sedimentos se han estudiado mediatne analisis micropaleontologico de ostracodos, con el objetivo de reconstruir la evolucion paleoambiental de esta zona. Estos conjuntos de microfauna integrados con el analisis de las facies sedimentarias permiten concluir la existencia de lagos con un nivel acuifero de pocas decenas de metros. Se concluye que existio un cambio en las condiciones quimicas de las aguas, que evolucionaron a partir de oligohalinos y sin gas a mesohalino y sulfatadas. EnglishThe siliclastic and carbonate deposits are interbedded in the Villadiego area (Miocene, NE Duero Basin). They have been subdivided into two high-rank depositional sequences: DDS and CDS. The sedimentary analysis of these units and the study of the microfauna content, mainly ostracods, led to the identification of lacustrinefluvial interaction systems. The sedimentary characteristics reveal the existence of fluvial systems of gravel, flood plains and lacustrine systems that were interconnected and intimately related in north-south direction. In the sedimentological analysis, thirteen types of fluvial and lacustrine lithofacies and six genetic facies associations were recognized. The top of DDS is the result of lake level risings. The CDS shows a deepening-shallowing cycle. The ostracod micropaleontological analysis of the sediments have been studied, with the aim of reconstructing the palaeoenvironmental evolution of this area. These microfauna assemblages integrated with the analysis of the sedimentary facies allowed to conclude the existence of lakes with a water-bearing level of few tens of meters. A change in the chemical conditions of the waters, which evolved from oligohaline and carbonated to mesohaline and sulphated is concluded.