Response of Plio-Pleistocene alluvial systems to tectonically induced base-level changes, Vera Basin, SE Spain

Tectonics are perceived to be a major control on the positioning and long-t erm evolution of alluvial systems. By increasing slope gradients through up lift and tilting, or by changing local base-level, incision can be stimulat ed leading to a switch in the active area of sedimentation. An example of s uch a sedimentary response to tectonic activity is provided by well-exposed late Plioccne/early Pleistocene alluvial sediments of the Salmeron Formati on from the western margins of the Vera Basin, SE Spain. Early stage palaeo geographic reconstructions demonstrate the occurrence of two alluvial fan b odies with distinct palaeocurrent and provenance signatures that suggest se diment source areas from the north (Sierra Lisbona) and south (Sierra de Be dar) of the study area. Late stage reconstructions suggest fan abandonment and indicate the occurrence of a braided river system sourced from the Sier ra de Bt dar in the south. Proximal parts of this braided river are incised by up to 100 m into underlying fan sediments sourced from the Sierra de Be dar. In distal areas, incision is negligible and the braided system forms a conformable sedimentary succession with underlying fan sediments sourced f rom the Sierra Lisbona. The switch from alluvial fan to braided river sedim entation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected th e Vera Basin during the early Pleistocene. Extensional faulting resulted in uplift and subsidence, leading to localized tilting of depositional surfac es in distal areas of the southern fan. Increased sl:ream power resulted in headward incision through mid and proximal fan areas. Once the system beca me fully trenched distal base-level controls became effective in generating an enlarged catchment area and continuing incision. The resultant changes dramatically increased both sediment and water discharge to the alluvial sy stem and a switch to braided river sedimentation.