Alluvial Fan Development and the Glacial-Glaciofluvial Cycle, Nenana Valley, Alaska

Alluvial fans located downstream from Pleistocene glacial margins in the Nenana Valley of Alaska began to form immediately after outwash deposition ended. Each fan constructed by a major tributary stream rests conformably on the outwash surface of the preceding glacial episode. Outwash is unweathered where it is covered by thick fan alluvium, indicating that no significant time break separated deposition of the two material types. Except for a thin upper zone, development of each fan was completed before trunk-river entrenchment occurred prior to the subsequent glacial event. It is possible that fans were constructed in a relatively short "paraglacial" interval when the effects of a nearby glacier were still being felt in the tributary basins. A complete fan sequence in any tributary valley near but beyond the downvalley limits of glaciation mirrors the mountain glacial sequence and can be used as a first approximation of the number of glacial episodes.