Geomorphic and geologic controls on alluvial-fan processes along the Coastal Cordillera (northern Atacama Desert, Chile)
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Interactions between alluvial fans and their drainage basin have been studied in the literature, but several questions remain unanswered. In particular, understanding which controls determine dominant depositional processes on alluvial fans has proved quite difficult. The extremely rare runoff and total absence of vegetation in the Coastal Cordillera of the Atacama Desert (northern Chile) offer perfect conditions for the analysis of geomorphological and geological controls on alluvial fans and drainage basins. In this study, a complete analysis has been carried out on all the geomorphic and geological characters of alluvial fans developed along the Coastal Cordillera. Both coastal fans (37) and fans located farther inland (10) have been investigated for 39 different geomorphological and geological variables. The number of examined variables is considerably higher than in previous studies. Analyses have been conducted by integrating data from satellite images, a geological map of Chile, and elaborations on a Geographical Information System (GIS). Five principal types of fan-surface morphologies have been distinguished in the Coastal Cordillera, controlled by primary depositional (sediment-gravity and fluid-gravity) processes and secondary reworking processes. Since climate, tectonics and catchment geology (lithology) are generally similar for all of the studied fans, catchment geomorphology is probably the ultimate control on different types of depositional processes and fan-surface morphologies. Mass-flow-dominated fans are characterized by small fan areas, steep fan slopes, small basin areas, great basin slopes, high relief, high basin Melton-ratio, low highest stream-order, low BA/BP ratio, a low sinuosity for the feeder channel and stepped hypsometric curves. Deposits are also important to differentiate between types of fan-surface morphology, but visual inspection by GIS fails when surfaces are reworked. Based on statistical analysis, the main discriminants between different types of surface morphology are, in decreasing order of importance: the BA/BP-ratio, basin slope, basin area, drainage density, fan radius, fan slope, fan width and fan area. This means that for alluvial fans in the Coastal Cordillera the stage of catchment development is very important, since larger, well-developed catchments have a more pronounced round shape (high BA/BP-ratio), and smaller undeveloped basins have a rectangular shape (low BA/BP-ratio). The most influential geomorphological relationship is between basin slope and drainage density.