Flow events on a hyper-arid alluvial fan: Quebrada Tambores, Salar de Atacama, northern Chile
Anne E. Mather, Adrian Hartley, 2005. "Flow events on a hyper-arid alluvial fan: Quebrada Tambores, Salar de Atacama, northern Chile", Alluvial Fans: Geomorphology, Sedimentology, Dynamics, A. M. Harvey, A. E. Mather, M. Stokes
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The Tambores alluvial fan is located within the hyper-arid Atacama Desert of northern Chile. We examine evidence of the range of flow processes operative in this environment from a combination of Pleistocene–Holocene fan deposits and a recent (2001) flood event (16 m3 s–1) in the fan feeder channel and upper alluvial-fan area. The field evidence suggests that peak flows recorded in the older deposits generated extensive sheetflood events dominated by antidune deposition in the upper fan area. These extreme, supercritical flows were generated by floods with sustained high sediment and water discharges and high stream power. Easily erodable alluvial source materials ensured high sediment discharge could be maintained within flood events. High stream power was ensured as a function of the tectonically exacerbated gradients within the source area. The 2001 event indicates the rapid rheological changes that can occur within an individual flood event, ranging from hyperconcentrated streamflow to mudflow. The flow deposits vary little in maximum clast size either between the varying flood events in the upper fan area, or down the fan gradient. This is due to a limited calibre of sediment being produced from the source area. The study highlights: (1) the range of flow rheologies that can be generated from a hyper-arid catchment both within and between flood events of varying magnitude and the associated difficulties in generating a reliable stratigraphy from the resultant deposits; (2) the high stream power and sediment discharge associated with major flood events and thus the nature of flood hazard in the catchment and on the fan; and (3) the limitations of sedimentological information such as maximum clast size as an indicator of peak flow characteristics in ancient deposits.
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Alluvial Fans: Geomorphology, Sedimentology, Dynamics
Alluvial fans are important sedimentary environments. They trap sediment delivered from mountain source areas, and exert an important control on the delivery of sediment to downstream environments, to axial drainages and to sedimentary basins. They preserve a sensitive record of environmental change within the mountain source areas. Alluvial fan geomorphology and sedimentology reflect not only drainage basin size and geology, but change in response to tectonic, climatic and base-level controls. One of the challenges facing alluvial fan research is to resolve how these gross controls are reflected in alluvial fan dynamics and to apply the results of studies of modern fan processes and Quaternary fans to the understanding of sedimentary sequences in the rock record. This volume includes papers based on up-to-date research, and focuses on three themes: alluvial fan processes, dynamics of Quaternary alluvial fans and fan sedimentary sequences. Linking the papers is an emphasis on the controls of fan geomorphology, sedimentology and dynamics. This provides a basis for integration between geomorphological and sedimentological approaches, and an understanding how fluvial systems respond to tectonic, climatic and base-level changes.