Adrian M. Harvey

Conventional sedimentological models of alluvial fan sequences tend to be based on fans in subsiding sedimentary basins. Fans in uplifting terrains are rather different. We examined the sequence of fan delta and alluvial fan deposition from the Pliocene to the Late Quaternary in the uplifting terrain of the Neogene sedimentary basins of Almería, Betic Cordillera, SE Spain. There were three phases of fan deposition, each yielding particular morpho-sedimentological characteristics and each having distinctive spatial distributions. The first phase was the Pliocene fan deltas, deposited where the larger river systems entered the receding early Pliocene seas. This phase was followed during the late Pliocene–Early Pleistocene by a phase of extensive alluvial fan deposition extending from the mountain fronts to the basin centres. Continued uplift occurred during the Pleistocene, resulting in incision of the drainage network and dissection of the Plio-Pleistocene fan systems. The third phase, during the Quaternary, involved much smaller mountain front alluvial fans set well below the Plio-Pleistocene fans, except in local areas of tectonic subsidence where extensive fans were formed. The older fans exhibit stacked sedimentary successions with very little evidence of sustained vertical incision. However, the Quaternary fans show much evidence of sustained incision between major aggradational phases, often resulting in inset rather than stacked stratigraphic relationships. We interpret this to imply that, although the tectonic context was the primary temporal control on variations within the older fan successions, within the Quaternary fans the overall tectonic control has been modified by responses to the Quaternary climatic sequence.

Alluvial fans and debris cones link two zones of the fluvial system (e.g. hillslope gully systems to stream channels; mountain catchment sediment source areas to main river systems or to sedimentary basins) and therefore have important coupling or buffering roles. These roles may be both functional and preservational. The functional role includes debris‐cone coupling, which controls sediment supply from hillslope gully systems to stream channels, influencing channel morphology. Coupling through larger alluvial fans, expressed by fanhead trenching, causes a distal shift in sedimentation zones, or when expressed by through‐fan trenching, causes complete sediment by‐pass. The preservational role stems from the fact that fans and cones are temporary sediment storage zones, and may preserve a record of source–area environmental change more sensitively than would sediments preserved further downsystem. Fan coupling mechanisms include distally‐induced coupling (basal scour, ‘toe cutting’, marginal incision) and proximally‐induced coupling (fanhead and midfan trenching). These mechanisms lead initially to partial coupling, either extending the immediate sediment source area to the stream system or shifting the focus of sedimentation distally. Complete coupling involves transmission of sediment from the feeder catchment through the fan environment into the downstream drainage or a sedimentary basin. The implications of coupling relate to downstream channel response, fan morphology, sedimentation patterns and vertical sedimentary sequences. Temporal and spatial scales of coupling are related, and with increasing scales the dominant controls shift from storm events to land cover to climatic and base‐level change and ultimately to the relationships between tectonics and accommodation space. Finally, future research challenges are identified. Modern dating techniques and sophisticated analysis of remotely sensed data can greatly improve our understanding of fan dynamics, and should lead to better cross‐scale integration between short‐term process‐based approaches and long‐term sedimentological applications, while maintaining high quality field‐based observations. Copyright © 2011 John Wiley & Sons, Ltd.