The fluvial architecture of the upper Buntsandstein in the Iberian Basin, central Spain

Abstract

ABSTRACTThe Cercadillo Sandstone and Siltstone (CSS) Formation forms the upper part of the Buntsandstein fluvial facies in the western margin of the Iberian Basin. Sedimentation was controlled by two major pre‐Triassic tectonic systems: the Iberian Ranges System (NW‐SE) and the Central System (NE‐SW). Fluvial sedimentation during this period took place in coexisting braided and meandering channels, together with associated ephemeral episodes. This unusual association was due to a combination of structural control by the main normal faults coupled with large discharge fluctuations.The CSS Formation consists of two major fluvial sequences with very similar vertical patterns comprising low sinuosity stream deposits that pass upwards into high sinuosity deposits associated with extensive floodplain sediments. A widespread calcrete horizon is developed at the top of the lower sequence and a surface with pedogenic features is present at the top of the upper sequence. These horizons are interpreted to be the result of subaerial exposure during a period of no subsidence and equilibrium in the erosion‐sedimentation processes in the area, probably due to quiescent periods on the master faults that delineate the basin.Minor changes within the sequences are probably due to discharge fluctuations of a climatic origin whereas the general vertical trend of the sequences is suggested to be tectonically controlled. Palaeocurrent data show a general flux of sediments to the south‐east, parallel to the axis of the Iberian Basin, but the final part of the upper sequence palaeocurrents to the north‐east are interpreted to indicate a local reactivation of secondary, transverse faults in the south‐east part of the study area.