Alluvial fan types, distribution, and formation: a global perspective

Berlin

Bornträger

2012

95-142

Artikel aus einer Zeitschrift

200212282

Zeitschrift für Geomorphologie 2024, 64(2) : print & online : A Journal recognized by the International Association of Geomorphologists (IAG) ; 64 (22024) , 95-142

Alluvial fan development is sensitive to environmental change, and thus, alluvial fans provide essential archives for reconstructing Quaternary paleoenvironmental conditions, notably climate, hydrology, and tectonics. Although alluvial fans have been studied across the globe for over a century, there is yet to be a unifying scheme/framework or model to view their complete variety and modes of formation and distribution. Reviewing the global spatial and temporal range of alluvial fan types and data from selected key regions allows us to examine their varied geomorphology, formation, and distribution and hence aids in their application for Quaternary climate and environmental change studies. This approach indicates that three main regimes for alluvial fan geomorphology and formation are evident: Type I) simple small-scale alluvial fans that are limited in size and extent (radius a few km) alluvial and megafans (radius >20 km) with an increasing influence of tectonics and ages going back into the Neogene. While small-scale alluvial fans are created by short-term events (10–1 to 10+1 years), mesoscale and macroscale alluvial fans develop over 104–5 year timescales. Type I alluvial fans are prevalent in high-latitude and high-altitude environments but may occur in numerous 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 tectonic uplift and faulting. 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. Autocyclicity also plays a role in alluvial fan development by adjusting to changing sediment sources and deposition and changing channel slopes. 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, weathering, and sediment flux. 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 associated with Pleistocene glaciofluvial extensive outwash plains. In semi-arid/arid cratonic and pericratonic regions where pediments or bajadas dominate, alluvial fans are less evident, and because these transition from Type III, we called them Type III’.
Keywords: alluvial fans; bajada; aggradation; deposition; distributed fluvial systems; tectonics; basins; cratons