A conceptual model for alluvial fan formation and development 

Abstract

The development of alluvial fans is sensitive to environmental change and, thus, alluvial fans provide essential archives for reconstructing Quaternary paleoenvironmental conditions, particular climate, hydrology, and tectonics. Although alluvial fans have been studied across the globe for over a century, there is no unifying scheme/framework or model to consider their complete variety and mode of formation. By reviewing the global spatial and temporal range of alluvial fan types and data from selected key dryland regions, we are able to develop a conceptual scheme/framework for their geomorphology and formation, and thus aid in their application for Quaternary climate and environmental change studies. This approach suggests that there are three main regimes for alluvial fan geomorphology and formation: Type I) microscale mountain alluvial fans, small in size and extent (radius < a few 100 m); Type II) mesoscale (radius a few km) and megafans (radius >20 km) mainly associated with large-scale Cenozoic mountain building. While small-scale alluvial fans are created by short-term events (10-1 years), mesoscale and macroscale alluvial fans develop over 104–5-year time scales. Type I alluvial fans are prevalent in high-latitude and high-altitude environments but may occur in many periglacial, semi-arid, and arid environments. In mid-latitude drylands, Type II mesoscale alluvial fans and bajadas are one of the most common fluvial systems and landform features, where they are frequently associated with steep mountain fronts, closely related, and often modified by active tectonics. Their formation occurs in different settings characterized by distinct relationships between sediment production and transport capacity. Although governed primarily by the climatic setting, bedrock geology can dominate, especially for sediment production. Periglacial sediment production (frost weathering) appears to be a major factor in their development, and provides an abundant sediment source during periods of climatic instability; hence climatic and weathering cycles are important, particularly in high mountain and high-latitude environments. Examples of the Quaternary climate influence on alluvial fans in drylands include those of the American Southwest, Mongolia, Iran, Morocco, Northern Sahara, Argentina, Chile, Namibia, and Australia. In these regions, alluvial fan development can be linked to climate variability on glacial-interglacial cycle timescales or expressed in late Quaternary pulses of increased humidity. For instance, they are common in glaciofluvial settings and periglacial areas, where streams have high bedload and transport capacity depending on the sediment supply and discharge by snow and ice dynamics. Type III alluvial fans are mainly associated with large-scale (mountain belts) Cenozoic tectonics and active uplift and are usually the oldest alluvial fans and for the most part inactive. Other Type III alluvial fans are also associated with Pleistocene glaciofluvial extensive outwash plains. In semi-arid/arid cratonic and pericratonic areas where pediments dominate alluvial fans are less evident, and because these transition from Type III, we have called them Type III’.