Infill architecture of a nonmarine half-graben Triassic basin (central Spain)

Upper Permian to Middle Triassic continental red beds were studied using architectural-element analysis in a selected area in Central Spain where the outstanding quality of outcrops allows a three-dimensional study. These red beds were deposited in a tectonically active half-graben basin with episodically changing rates of basin subsidence. Seventeen architectural elements (channel belt and overbank elements) were recognized, described in detail, and interpreted. Twelve channel-belt elements and five overbank elements are present in the Buntsandstein. They combine in nine different alluvial systems with diverse bed-load channel type and discharge regime (gravel bed-load braided, mixed-load braided, three different sandy braided, intermediate sinuosity, high sinuosity, ephemeral, and alluvial fan). Not only the sizes of the different elements but also the scale of the complete alluvial systems were emphasized in this study. The differential tectonic, vertical and lateral evolution in the area was the main control on basin asymmetry, channel interconnection, paleocurrent pattern, and consequently the fluvial architecture. The asymmetry of the basin, together with the presence of antithetic and synthetic fault systems, caused the fluvial system to migrate away from the footwall scarp, to a more eastern position, where the lowest area was located. As the effect of the initial increase in subsidence was compensated by the sedimentation, the axial southeastern trend then became reestablished.