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

Abstract

The Plio-Pleistocene sedimentary rocks of the Sorbas Basin, SE Spain are used to reconstruct the palaeogeography, palaeoclimate and active tectonics operative 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 Sierra Alhamilla and (2) a more basinal, smaller catchment developed to the north of the structural lineament. The alluvial systems show evidence for sheet flooding, channelized flow, and overbank sedimentation in distal and marginal areas and are interpreted as fluvial distributary systems. The deposits were subjected to syn-sedimentary folding which exerted a strong control 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 eventual cessation of deposition from the larger catchments is explained by their capture by an aggressive external drainage developing south of the structural lineament. The switching off of sediment supply from the larger catchments, coupled with high subsidence rates in the sediment dispersal area enabled 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 significance of river capture in re-routing both sediment and water discharge between sedimentary basins (the Sorbas Basin lost 15% of its original sediment and water budget to the Lucainena fan delta and associated coastal system of the Carboneras Basin to the south) and its subsequent effect on the pirated area (reduction in rates and changes in style of deposition). These latter factors outweighed the direct impact of tectonics and climate on the later development of the studied alluvial systems. The study emphasizes that to maximize the effects of sediment re-routing on actively aggrading alluvial systems (in the sedimentary basins) the positioning of the capture point is crucial and is most effective where (1) the sediment supply areas (mountain catchments) are pirated closest to their outlet (mountain front) into the receiving sedimentary basin and (2) the pirating drainages are external to the sedimentary basin.