Impact of headwater river capture on alluvial system development: an example from the Plio-Pleistocene of the Sorbas Basin, SE Spain

The Plio-Pleistocene sedimentary rocks of the Sorbas Basin, SE Spain are us ed to reconstruct the palaeogeography, palaeoclimate and active tectonics o perative in the Late Pliocene-Early Pleistocene. The deposits are generated From two distinct source areas. These are(1) relatively larger catchments developed across a structural lineament situated in the Foothills of the Si erra Alhamilla and (2) a more basinal, smaller catchment developed to the n orth of the structural lineament. The alluvial systems show evidence for sh eet flooding, channelized flow, and overbank sedimentation in distal and ma rginal areas and are interpreted as fluvial distributary systems. The depos its were subjected to syn-sedimentary folding which exerted a strong contro l on the general topography at the time of alluvial system development. The balance between the accommodation space created by the tectonics operative over the Plio-Pleistocene. coupled with the sediment discharge, determined the alluvial system morphology and sedimentological architecture. The even tual cessation of deposition From the larger catchments is explained by the ir capture by an aggressive external drainage developing south of the struc tural lineament. The switching off of sediment supply from the larger catch ments, coupled with high subsidence rates in the sediment dispersal area en abled the smaller alluvial system, still connected to its source area north of the lineament, to expand. Eventually the remaining catchment areas were also reduced by continued river capture. The study emphasizes the signific ance of river capture in re-routing both sediment and water discharge betwe en sedimentary basins (the Sorbas Basin lost 15% of its original sediment a nd water budget to the Lucainena fan delta and associated coastal system of the Carboneras Basin to the south) and its subsequent effect on the pirate d area (reduction in rates and changes in style of deposition). These latte r factors outweighed the direct impact of tectonics and climate on the late r development of the studied alluvial systems. The study emphasizes that to maximize the effects of sediment re-routing on actively aggrading alluvial systems tin the sedimentary basins) the positioning of the capture point i s crucial and is most effective where (1) the sediment supply areas (mounta in catchments) are pirated closest to their outlet (mountain front) into th e receiving sedimentary basin and (2) the pirating drainages are external t o the sedimentary basin.