Quaternary telescopic-like alluvial fans, Andean Ranges, Argentina
The largest rivers that drain the Argentine Andean Ranges are characterized by incised valleys in high mountains and by a variety of Quaternary terraces. The terraces display a fan geometry with the apex located upstream of a tributary junction. Their convex-up morphology suggests that these terraces are related to a series of alluvial fans developed where the tributaries join the main river. The succession of alluvial aggradation and degradation is controlled by local base-level variation conditioned by temporary lake development in the main river valley. All these factors give rise to the inset segmented (terraced) morphology of the fan surfaces, yielding a telescopic-like relationship. The variation in the morphology and number of terraces suggests that they are not controlled by a general/regional base level. Neither tectonic activity nor significant climatic changes account for the alluvial fans at the confluences of the tributaries and the main river. Significant variations in rainfall or thunderstorms induced by the El Niño Southern Oscillation (ENSO) could explain the genesis of these telescopic-like alluvial fans.
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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.