Braided stream and flood plain architecture: the Rio Vero Formation, Spanish Pyrenees

Early- to middle-Miocene fluvial sandstones of the Rio Vero Formation were studied, in an area around the town of Barbastro, south central Pyrenees Sp ain. The outstanding quality of outcrops in this area allows a three-dimens ional study of architectural elements. Six architectural elements are recognised, described in detail, and interpr eted from three key localities. Seven main lithofacies were identified and sub-divided into gravelly, sandy and fine-grained lithofacies. The architec tural elements and lithofacies have been combined with a hierarchy of depos itional bounding surfaces to fully interpret the evolution of the depositio nal system at the meso- and macro-scale. Not only the different architectur al elements and lithofacies of the complete braided fluvial system, but als o the lateral variation of the architectural elements were emphasised in th is study. Differential tectonic movements, seasonal climate change, and the ir effect on vertical and lateral evolution of the area were the main contr ol on basin sedimentation, channel interconnection, palaeocurrent patterns, and consequently the fluvial architecture. The presence of lateral ramp anticlines caused the fluvial system to be lat erally restricted, with the main channel-belts being located in the areas o f highest subsidence and lowest topography. Intervening topographic highs a cted as both flood plains and lateral barriers between the main channel sys tems. The proposed depositional model comprises broad, low-sinuosity, peren nial, but seasonal moderate-energy streams. The sandstone architecture is d ominated by channel-fill and sheet sands, and associated simple and more co mplex bars. Adjacent to the main channel-belts fine-grained sandstones, sil tstones and immature paleosols occur. The along-strike relationship between major fluvial systems and their outle ts into a foreland basin has important implications for the infill of the b asin and the modelling of fluvial systems along mountain belt fronts. (C) 2 001 Elsevier Science B.V. All rights reserved.