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1 Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA, UK (e-mail: m1stokes{at}plymouth.ac.uk)
2 Department of Geographical Sciences, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA, UK
Scientific editing by Peter Haughton.
Tectonics are perceived to be a major control on the positioning and long-term evolution of alluvial systems. By increasing slope gradients through uplift and tilting, or by changing local base-level, incision can be stimulated leading to a switch in the active area of sedimentation. An example of such a sedimentary response to tectonic activity is provided by well-exposed late Pliocene/early Pleistocene alluvial sediments of the Salmerón Formation from the western margins of the Vera Basin, SE Spain. Early stage palaeogeographic reconstructions demonstrate the occurrence of two alluvial fan bodies with distinct palaeocurrent and provenance signatures that suggest sediment source areas from the north (Sierra Lisbona) and south (Sierra de Bédar) of the study area. Late stage reconstructions suggest fan abandonment and indicate the occurrence of a braided river system sourced from the Sierra de Bédar 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 Bédar. In distal areas, incision is negligible and the braided system forms a conformable sedimentary succession with underlying fan sediments sourced from the Sierra Lisbona. The switch from alluvial fan to braided river sedimentation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected the Vera Basin during the early Pleistocene. Extensional faulting resulted in uplift and subsidence, leading to localized tilting of depositional surfaces in distal areas of the southern fan. Increased stream power resulted in headward incision through mid and proximal fan areas. Once the system became 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 system and a switch to braided river sedimentation.
Keywords: SE Spain, alluvial fan, braided river, tectonics, base-level.
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